Geology

Sculpting the Parkers Creek Preserve

October 21, 2021 By crm

What made our ravines, stream valleys, cliffs, ridges, and that mound rising above the salt marsh?

By Peter Vogt, Founding Member

Although part of the mostly gentle Atlantic and Gulf Coast Coastal Plain, the Parkers Creek Preserve (PCP) is scarcely a plain. That’s obvious even to first time ACLT hikers or Parkers Creek canoeists. Our preserved land rises locally to more than +160 ft above sea level, and some ravines are more than 100 ft deep. Adding canopy trees 75 to 140 feet high makes the land look even higher, especially when viewed from out in the Chesapeake Bay. The Calvert Cliffs form a formidable 50-100 ft high eastern rampart for the PCP. The preserve owes its dissected topography—at once as ideal for ecological diversity and nature hikes as it is challenging for farming or development—to the proximity to the Chesapeake of lands well above sea level. Steep gradients empower running water with the gravitational energy to sculpt the land. While Minnesota may be the “Land of 10,000 lakes”, are we the “Land of 10,000 ravines”?

Not just an ecologically resilient ‘bio-preserve’ and nature park, the PCP is also a museum of geomorphology. That’s the study of landforms and the processes that shaped and are still shaping them: the topographic texture on which our living green carpet is draped. We have some interesting land forms, like that mysterious forested mound rising 43 ft above the Parkers Creek salt marsh. Hint—it was not built by ancient Native Americans or modern developers. Parkers Creek itself is a geomorphic oddity—flowing into the Bay when it was originally the middle part of a longer Battle Creek, flowing into the Patuxent.

Mankind is increasingly the main geomorphic agent (think bulldozers), but except for increased erosion caused by pre-modern farming, the PCP has been shaped by nature. However, nature tends to be a slow and patient sculptor of land—the morphing is too slow on our human time scales. In the PCP area, only the Calvert Cliffs and beaches change fast enough for us to notice.

Mapping and Displaying topography

Topography can be shown in different ways. A simple 3-D sketch or a photograph-ideally taken when the leaves are off the common deciduous trees and with some snow on the ground, can provide a local view. Contour maps became common years ago but may be greatly in error! An elevation contour is just a line on a map connecting points of the same elevation. ACLT trail maps have generally shown elevation contours—mostly at 20 or 30 ft contour intervals. Finer contour intervals would show more detail but too much clutter for a paper brochure. To see those details, go online and zoom down! Modern digital representations of topography—as computer illuminated and colored “hill-shade’ images–offer photographic realism, but elevations can’t be ‘read’ as on a contour map. They are useful to hikers but not as useful as contours. Hill-shade images are generally computed from LiDAR (Light Detection And Ranging) scanners deployed on manned aircraft or drones. A laser is scanned (swept) back and forth across the ground at right angles to the flight path and at high rates. Laser pulse reflection times from the ground back to the sensor are measured and converted. Calvert County was first flown with LiDAR in 2005. In my opinion the greatest potential value of LiDAR for ACLT would be mapping forest canopy height—by subtracting laser reflections from tree tops from the ground elevations. A measure of forest type, age and health.

However valuable contour maps are, we humans relate much better to maps showing the ‘culture’ we have written—and keep rewriting– on the landscape. For the PCP that’s mostly roads and trails. We can also relate to ACLT’s ‘virtual culture’—property outlines with diverse colors showing the status of land preservation. We follow the progress of ACLT land preservation on maps which have practically no visible expression on the ground.

I liken this culture (e.g., roads, trails, power lines, buildings, property boundaries, cleared fields, etc.) as well as our natural vegetative cover to a palimpsest. We humans ‘write’ our things on the land by erasing much of what nature has written. And then we may tear down historic buildings to make room for others. Other metaphors include black and white boards or painter’s canvases which, like the vellum or parchment (animal hide) of old– are just too valuable not to reuse. Of course the same is true for land—as our former MD Comptroller and Parkers Creek land owner liked to say about land “God made only so much of it.” Competing uses for the same land are a reason for land trusts like ACLT!

Nature slathers her corrugated canvas—the land– with thick ‘paint’—lush green in summer, more gray and brown in winter, sometimes even white. If you don’t like the ‘paint’ metaphor, how about a living carpet? While ACLT naturally concerns itself mostly with the ‘paint’ (or ‘carpet’), this article is about the underlying ’textured canvas’. But metaphors have limits—paint is not alive and does not derive its sustenance from the ‘canvas’. In fact the soil and subsoil form a living underground world of roots, fungi, bacteria, and burrowing critters.

While many ACLT members are familiar with PCP topography from hiking and canoeing, it’s challenging to orient oneself—can you locate your favorite ACLT trail?– on a contour map stripped of culture, particularly roads and trails. Of course we recognize Parkers marsh and the Calvert Cliffs, but where for example is the ACLT South trailhead or Double Oak Farm? To save readers the head scratching, I placed small yellow flowers at those three spots. The Yoe farmhouse sits near the headwaters of a major feeder ravine to Parkers Creek’s longest tributary. Double Oak is near the drainage divide (saddle) between the Horse Swamp creek watershed and a smaller tributary watershed further north. The ACLT South trailhead is appropriately very close to the drainage divide (interfluve) between the Parkers Creek and Governors Run watersheds whose preservation remains our land trust’s focus.

I also labeled that marsh mound (M), and drew some straight lines showing topographic trends of several ridges and valleys. (More about those later).

Land above +140’ is shown black and from +100 to +140 ft stippled. That helps readers orient themselves. A plain contour map-depending on how it is colored—a kind of natural Rohrschach test or paint- by- numbers painting. Coloring selected elevation intervals with a felt tip or crayon can be therapeutic— much slower but more exciting than coloring by computer graphics. Nowadays geomorphologists routinely use computer graphics with different combinations of illumination and coloring to tease out subtle patterns of interest.

When PCP topography is shown this way, it looks dramatic. But modeled to scale in plaster at page size, the relief would only be around 1/16” inch, just enough to feel if you ran your fingers over it.

PCP Ecosystem Diversity and Resilience: Shaped by Topography and Geology

Our topography alone creates ecosystem diversity. North- facing slopes have a cooler, moister microclimate, different from the drier hotter south-facing slopes. Soggy flat valley bottoms support ecosystems distinct from those on flat ridge crests. The salt marsh, freshwater tidal and non-tidal swamps add further diversity, together making the PCP a microcosm of the Chesapeake Bay.

However, PCP topography also has a geologic connection—important for understanding our ecosystems. Where streams dissect a stack of flat-lying sediments or sedimentary rock, the layering is exposed—though maybe draped by soil–in valley walls. Progressively older layers crop out (or “subcrop”) further down the slopes. Of course the younger and higher a layer, the more of it has been lost to erosion. Grand Canyon is a grand example, but the PCP is fundamentally the same phenomenon, but for the smoother and 50 times lower relief. In neither case are the layers perfectly horizontal, but that’s a good approximation.

In contrast with the Grand Canyon- where old rocks are exposed in every side canyon- in the PCP we only rarely see any exposures of the Miocene layers even on the slopes of ravines. The reason is obvious—our Miocene layers were never turned into hard sedimentary rock, as have the ancient layers forming Grand Canyon’s cliffs. However, Miocene fossils are sometimes found in Calvert County ravine bottoms. Groundhogs may dig out of their burrows bits of fossil shells which however eventually dissolve in our acidic soils.

Our Miocene layers are tilted (dip) to the southeast at a few feet per mile. The shell marl exposed to Chesapeake Bay waves at the base of the Governors Run cliff would likely be encountered in a borehole at ca. 15-20 ft above sea level around 120 ft below Double Oak farm, which is about 3 ½ miles northwest of Governors Run. Shell marl is sediment which contains lime—in our case fossil mollusk shells.

The oldest PCP sediments above sea level were laid down 15 or 16 million years ago—around the middle of the Miocene epoch of geologic time- and are exposed near the base of the PCP’s Calvert cliffs. Some of these layers are so cohesive they crack into blocks just as harder rocks would. The youngest of these marine sediments date from around 10 million years and in the PCP are exposed around 100 ft above sea level. A total of three shell marls occur within this 100 ft section and where they intersect the ravine slopes buffer our otherwise acid soils. This geologic buffering is the only way to explain the moderate pH of PCP streams—the pH is well above that of our rain. How well plants can utilize nutrients depends on soil pH! Tulip poplar, spicebush and pawpaw are common forest flora. Botanists have even spotted some PCP wildflower species otherwise known only from limestone substrates of the Appalachians.

Atlantic Retreat Is followed Potomac Advance

Around 8 million years ago the Atlantic Ocean retreated from what is now Southern Maryland. Relative sea levels likely fell, in part due to ice sheet expansion in Antarctica. From then until around 2 or 3 million years ago the Potomac and its tributaries flowed across here on their way southeast to the coast. The ‘fluvial’ (river deposited) sediments have more sand or even gravel, and very few fossils. In the PCP these “Upland Deposits” are present where our land is higher than about +100 ft (stippled or black).

Lacking shell marls and also low in nutrients, the Upland Deposits support flora which do well in more acidic soils—ACLT naturalists can attest that pine, mountain laurel and even oak species are generally restricted to our higher elevations. Sands winnowed from the Upland Deposits by streams work their way downstream and eventually replenish beaches.

The presence of Upland Deposits capping Calvert County shows that the Patuxent must have formed after the Potomac switched its Coastal Plain course at the Fall Line, where it today turns sharply to southwest. Before that switch, the river continued on its SE course it still has across the Piedmont. What caused this course switch (similar in sense to that of the Delaware and Susquehanna) is beyond the scope of this article, but it ultimately initiated formation of the Patuxent River and thus the stream layout on the present PCP.

The Upland Deposits were laid down on a gently sloping plain—like that along the US Gulf Coast today. Remnants of this ancient plain are scattered around in Calvert, including the PCP. The wide field at the intersection of Parkers Creek Road and MD 765 is a great example, and the gently rolling terrain around Double Oak and Holly Hill. Nearly level lands erode also, but only very slowly: Maybe just an inch per 1000 years! The original plain sloped very gently towards the east, but also covered what is now the Chesapeake Bay.

How PCP topography was born: Dissecting the Upland Plain by Headward Erosion

With the new Patuxent nearby, tributaries began to develop by headward erosion towards the west and east. Headward erosion means the tip or ‘head’ of a stream, ravine or gulley propagates upstream, i.e., ‘eats’ its way into the nearly level land. When past Calvert farmers plowed their fields right up to ravine edges, they soon noticed gullies growing into the fields and consuming their land.

As a valley or ravine lengthens, erosion of its margins cause them to widen. If there are any subtle remnants of the original SE flowing streams, I am not aware of them. Thus, the stream pattern we see today on the PCP began with streams flowing into the Patuxent starting very roughly a million years ago—accurate dating is challenging. The streams heading west towards the Patuxent flowed from an upland drainage divide out in (and above) what is the modern Chesapeake. This was the divide between the Patuxent and Susquehanna watersheds. Parkers Creek was the middle part of a longer Parker-Battle Creek. The upland once between the present mouth of Parkers Creek and that divide was lost to cliff erosion. The divide elevation is unknown but had to be above the +45 ft elevation of the remaining short segment –the ‘dry valley’ utilized by German Chapel Road–of the old Parker-Battle stream.

Stream Piracy–How the Chesapeake stole Parkers Creek from the Patuxent

The present PCP topography, dominated by the east-flowing Parkers Creek is of geologically more recent origin but was also caused by headward erosion. This ‘stream piracy’ or ‘inversion’ makes our land unusually interesting but is challenging to explain in words. Cartoons would be better, but a computer animation movie would be best. Any computer-savvy folks want to help me explain how Parkers Creek was stolen from the Patuxent? Such a movie would also cover the SE to SW course switch of the Potomac.

Anyway, here in words: We have had a succession of Chesapeake Bays, and each one caused more upland loss by cliff erosion. The penultimate Bay, ca. 125,000 years ago, had sea levels around 20 ft above today’s. When cliff erosion- perhaps from an even earlier Chesapeake Bay- first passed the old Patuxent-Susquehanna drainage divide, it was easy for an infant east-flowing stream to ‘eat its way’ upstream (headward erosion) along the channel of Parker-Battle Creek. Steeper gradients empowered young Parker’s Creek. The existing channel fill was soft and a valley already existed. How long it took for the new Patuxent-Chesapeake divide, i.e., the headwaters of Parkers Creek, to migrate to where it is today, crossed by German Chapel Road, is unknown. I estimate that the piracy began as late as 150,000 years ago— we humans already existed but had not left Africa.

Although inherited from a tributary to the Patuxent, the Parkers Creek watershed is more deeply dissected today than a million years ago. The upland plains have shrunk to scattered patches and the ravines have become deeper. Much of these changes have happened in the last 600 thousand years—with their large alternations of high and very low sea level.

Thus, all of the present Parkers Creek watershed once drained west into the Patuxent. The Parkers tributary streams of course did not change direction, and the overall pattern of streams and ravines has not changed. However tributary stream and ravine gradients have been increased—and are still increasing as the cliffs continue to migrate west, locally slowed by human shoreline protection.

The steep gradients give stream water more energy to cut into the substrate and cut inland by headward erosion. As the cliffs retreat westwards, the Chesapeake Bay steals land from the Parkers Creek watershed: The present strip of land about 500 to 2000 ft wide (the Warrior’s Rest cliffs and much of Scientists Cliffs) now drains directly into the Bay. For example, the north fork of Governors Run (colored yellow) flows south parallel and just 350 ft inland from the cliffs. Absent human tampering, shoreline westward retreat by the Bay would capture this fork in just around 2000 years. Whether rejuvenated ravines pirated from the Parkers watershed, or brand new, these short ravines make up for their tiny watersheds with high energy, locally even forming small waterfalls during cloudbursts. Arresting shoreline retreat won’t stop the Bay from stealing more land from the Parkers Creek and Governors Run watersheds.

Farming activities by English colonists and their American descendants accelerated erosion of the Upland Deposits, particularly by the headward erosion of ravines. Many of these formerly active gullies are now stably forested.

The Parkers Creek Marsh Mystery Mound

The most unusual topographic feature in the Parkers Creek Preserve is an elongated mound or island rising about 43 ft above from the surrounding salt marsh. Trees covering the mound may increase its total height to more than 100 ft. ACLT canoeists notice this on their first trip up the creek and might wonder if it was made by humans, for example by ancient Native Americans. However, there were no mound-building cultures here, and no evidence for an artificial origin. However, who’s to say the mound was never used as a lookout by scouts or hunters? The mound might have been made by past Parkers Creek meanders and meander cutoffs—forming intermittent oxbow ponds. Alternatively, it may have formed by shoreline wave erosion during times when there was no marsh and no barrier beach. While the marsh mound looks to be a remnant of a formerly longer marsh island spur ridge, it’s strangely oriented in another direction: NW-SE. This shape might reflect past meanders but alternatively was caused in the absence of marsh and barrier beach by shoreline erosion from Bay or lagoon waves eroding its shores from the northeast.

The PCP Is About Water

Water seems nearly the sole common denominator for what shaped and continues to shape the Parkers Creek Preserve (I said ‘nearly’—read on). Waters of the Chesapeake Bay gnaw at the cliffs slowly replacing our dissected uplands with a very gentle estuarine floor. Of course this only happens during interglacial warm periods with high sea levels, most recently the last 6000-7000 years. At other times the cliffs stay put as wooded escarpments.

Pore water trapped in Miocene cliff sediments helps erode them by freeze and thaw in winter. Rain water washes sediment into the ravines and then into the Bay. Protracted rains soak into the ground, causing landslides on steep slopes. Cohesive Miocene sediments are fractured, letting water seep along joints, helping loosen blocks but locally carrying dissolved iron down as cement to strengthen the sediments (cliffs north of Parkers Creek beach). All the sediments eroded from the PCP and carried to the Bay had already been carried there by water—the Miocene sediments in the Calvert Cliffs and present everywhere below about +100 ft were deposited from Atlantic Ocean waters. Sediments comprising the ‘Upland Deposits’ were brought here by the ancestral Potomac.

A Touch of Tectonics– Any PCP topography with structural control?

Is anything about our PCP topography NOT caused by the actions of water? Maybe so, but subtle. Stream patterns developing and growing on uniform sediments produce the familiar dendritic patterns like the veins in a leaf. The two large PCP tributary watersheds south of Parkers look pretty dendritic.

When geomorphologists look at accurate topographic maps, they keep an eye out for “structural control”. By that they mean faults or folds or even simple fractures (joints) that may have influenced the pattern of streams. For example, a fault in older deeper rocks might have become reactivated and propagated up into the overlying sediments and fractured them. The headward erosion of a stream would follow such a zone of weaker, more easily erodible sediment. Or, if groundwater had seeped down along a fault zone or joint system and deposited iron oxide, these sediments would be more resistant, avoided by a growing stream. In a landscape dominated as the PCP is with dendritic drainage patterns, a search for ‘structural control’ involves looking for longer straight valleys or ridges and regularities in trends and spacing. However not all faults are linear—a rising salt dome or a subsiding crater could have patterns of circular faults. If there is too much dendritic stream topography, structural control might not be recognizable, or people would argue about its existence. It’s a matter of ‘signal to noise’. We humans are great at pattern recognition, but we are also prone to see things not really there.

Let’s now first take some steps back and look at the geology of our region. Coastal Plain geology, even our dissected PCP, seems pretty boring compared to the western US. That’s because we sit on a passive continental margin, which, since plate tectonics separated North America from Africa around 175 million years ago has been very peaceful. Our margin is slowly subsiding and accumulating erosional detritus—sediments—eroded and weathered from the Piedmont and Appalachians. Under the PCP lies about a half mile of shallow marine or marsh sediments—the oldest below us were laid down around 125-150 million years ago. That’s the long term subsidence—averaging about one foot per 50,000 years. A rate trivial compared to our present sinking rate—one foot per 300 years—the delayed response to the “recent” great Laurentide ice sheet and its melting. The part of our relative sea level rise we can’t change but which will continue at slowly decreasing rates (discussed in Quo Vadis article).

However passive the continental margin, our plate interior has been under horizontal compressive stress beginning soon after we separated from Africa. Due to the slow stress buildup, our strong crust, and the stress being compressive, it takes many centuries or even millennia before there is enough stress to break the rock and thereby produce an intraplate earthquake, but when one happens, it is felt over a much wider area (less attenuation of seismic waves in old crust) than a comparable one on the West Coast.

The 2011 Louisa, VA quake (magnitude 5.8) was clearly felt here in Calvert, as was the 7.0 shock that destroyed Charleston, SC in 1886. If cliffs are shook in a strong earthquake, landslides can happen, so seismicity can be a geomorphic agent. As far as I know, the Louisa shock did not cause landslides along our Calvert Cliffs—it was close but not close enough. The Charleston quake was much too far away but I doubt anyone checked the Calvert Cliffs for new landslides.

The Louisa shock occurred in the Piedmont and the Charleston one on the Coastal Plain, but they both involve reverse motion on steeply dipping reactivated faults. Reverse fault motion involves one side shoved up over the other. Closer to Calvert, the Stafford faults in Virginia and the Brandywine fault system under Charles County, as well as the fault under Louisa, are also reverse faults. They most all trend (strike) NE to NNE. In all these cases, ancient preexisting faults were reactivated. The faults remained as zones of weakness—where repeated breakage is most likely. (Anyone who breaks apart wooden furniture notices that breaks often follow old glue lines or other joinery). The faults under Louisa and Charleston had been normal faults (extension) as preludes to formation of the Atlantic by plate tectonics. Long before that, the same faults had been involved in the continental collisions of the Appalachian orogenies (mountain building episodes) that closed an earlier ocean from 500 to 250 million years ago.

There can be little doubt that the ancient crystalline and metamorphic rocks a half mile and more below Calvert County are criss-crossed by earthquake faults and other structures. While modern technology is more than adequate, who would pay for a number of boreholes to learn more? Whether, where or how often any of these faults could rupture again is unknown, but if faulting happens, it must be very rare. However, the buried Taylorsville Basin across northern Calvert County, formed around 225 million years ago, is bounded on one side by a NNE trending fault similar in age and trend to those mentioned above which have ruptured in more recent times. The Miocene sediments exposed in the Calvert Cliffs (laid down from 18 to 8 million years ago) are not offset by faults except at one site along the southern cliffs. A high angle reverse fault has offset the Miocene layers so must have been active until at least 8 million years ago. It is unknown whether younger Upland Deposits were also offset. The trend of this “Moran’s Landing Fault’ remains unknown, but given its location between the LNG terminal and the nuclear power plant should be investigated.

Even in the absence of faults (with displacements) crustal stress can create patterns of simple cracks, called joints. Topography can offer clues—any evidence for ‘structural control’ in PCP topography? I think so, but the evidence is subtle. Some of our lower Parkers Creek watershed topography, although much carved out by water, seems too linear to have been created entirely by water- I drew ridge lines in red and valleys in blue. Possible structural control guiding the original courses of streams. There appear to be two main trends, at right angles to each other: A NNE trend and an ESE trend. These lineations are somewhat regularly spaced—about 1500-2000 ft apart. One of the NNE ineations is that half-mile long ridge just east of Double Oak.

One NNE ridge trend happens to align with the Parkers mound and associated spur, both relics of former higher and wider features. Maybe the location of this mound was not solely the work of stream erosion and meanders. When the most conspicuous (and most easterly) ridge trend is extrapolated, it grazes the cliffs north of Parkers Creek. The sediments in the lower parts of those nearly vertical cliffs are very cohesive, with joints locally cemented by iron oxide. A clue that dissolved iron percolating down along joints and precipitating in the Miocene sediments may have been a reason for this ridge. Maybe.

Tectonic origin of regular cracks (fractures or joints) in the cliffs?

Any independent evidence for NNE and/or ESE trending structural trends? Maybe. The lower, most cohesive layers in the Calvert Cliffs exposed in the PCP just south of Parkers Creek beach have been fractured by some process. Modern cliff erosion is in the form of blocks caving off along those fractures. There are numerous such joints spaced regularly around one foot apart, steeply dipping. The trends (strikes) are hard to measure accurately but appear to be E-ESE. These joints may have formed by thermal contraction and extension during the annual air temperature cycle, but if so would not continue deep into the cliff. If the joints are tectonic, they would extend through the ground under the cliffs. Some of the block failures may be controlled by NNE trending joints, but any cliff naturally fails along cliff-parallel joints. Whether or not these two sets of joints and resultant block collapses are partly of tectonic origin, they represent geomorphology processes working fast enough for humans to study.

It’s likely but not proven that sediment layers under the PCP have been fractured (by long term horizontal intraplate stress) along the well known regional NE to NNE structural trends. When our tributaries to Parker-Battle creek first formed, they would likely have propagated in one direction by headward erosion along zones of sediment weakness such as fractures (joints) or even, at depth, minor faults.

Quo Vadis, Parkers Creek Preserve

June 4, 2021 By crm

Will rapid sea level rise turn our marsh to mud flats and shallow lagoon?

By Peter Vogt, ACLT Founding Member

In the Spring 2021 Newsletter (pg.8), I speculated on the future of the ACLT as the successful land conservancy it has become. Now about the future of the Parkers Creek Preserve (PCP), its dramatic habitat diversity, an island in a vast urban-suburban sea. Our small but growing oasis is protected into the indefinite future for nature and passive human recreation. The protection is by legal, not physical fencing, and how we manage this oasis. Ecology is the response of life to physical forcing—notably climate. Absent human influences, it would be easy to predict the climate, sea level and ecology even millennia from now.

As is, what will happen especially to the salt marsh and adjoining freshwater swamps in the coming decades and centuries is awash in uncertainty. How fast will humans reduce greenhouse gas emissions? How fast and how much will climates change, and how much and how fast will sea levels rise—largely by ice loss in Greenland and Antarctica? How will tidal Parkers Creek and the PCP respond to those distant and global processes? How will PCP biota respond to warmer climates and higher CO₂? How much will the oceanic and thus the Chesapeake’s acidity increase, impairing carbonate shell growth by oysters and clams? How much will land-falling tropical cyclones intensify due to a warming Atlantic? Will land-fall frequency change? How might multidecadal variability change—confusing long term climate change predictions?

Instead of using fuzzy terms like ‘the middle of the next century’ I will hazard predictions for four “preservation milestone years.” That’s to sharpen our sense of past and future times, relating the ACLT to older Mid-Atlantic land preserves –all of which still exist today! The Boston Common (A, 1634) was the first urban park; Thomas Jefferson’s sale (B, 1815) of Virginia’s Natural Bridge as a ‘trust’; the oldest land conservancy (Trustees of Preservation, C, 1891); and the first Calvert nature preserve (Hellen Creek hemlocks, The Nature Conservancy, D, 1956). The ACLT is 35 years old this year, and will be as old as A in the year 2373, B in 2192, C in 2116, and D in 2051. While 2373 may feel unimaginably far in the future, that year is actually 35 years closer to us than 1634!

I’ll start by predicting that chemical pollution and its longer lasting effects will continue at least for some decades. Additional alien species will surely invade the PCP. These are biota not yet introduced, plus others, like fire ants, which will migrate up here, enabled by climate warming. If alligators get into Parkers Creek by 2192, would ACLT find them interesting or try to kill them off? Thousand Cankers Disease (fatal to black walnuts) is already in Cecil County, and the spotted lanternfly, chowing down on many tree species, is poised to invade from southeastern Pennsylvania. Insect populations are in decline—which would imperil bats and many of our insectivore song birds. However, future gene editing might safely deal with many pathogens, as it has with the chestnut blight. Given time, native plant species might evolve resistance while some native predators might evolve a taste for invasives.

As the Bay widens by wetland loss in Dorchester County, the increasing fetch would increase Calvert Cliff wave erosion, but warming would also decrease the effect of freeze-thaw. Mostly, climate change and sea level rise won’t have major effects on Calvert Cliffs shoreline retreat rates—see more below.

More frequent high rainfall events will increase slope erosion– but scarcely to high historical erosion rates from farmland. Increasing CO₂ will stimulate plant growth (including invasives and poison ivy) but make foliage less nutritious for insects.

Depending on future CO₂(plus other greenhouse gas) emissions, our climate by 2192 and especially 2373 would range from somewhat similar to today to tropical. After that, climates should stabilize and very slowly cool. My optimistic prediction for those distant years is a new Old-Growth forest similar but more bio-diverse than what hikers see today. Some American species will have migrated here from the coastal Carolinas or Georgia. (A pessimist might predict a species-poor environment dominated by invasives, with few trees growing large and old).

The most dramatic change in the PCP future may well be the transformation- by rapid sea level rise- of the salt marsh and adjacent freshwater swamp. They may turn into mud flats and then a shallow lagoon. A marsh doesn’t care how high sea levels will eventually get—provided the rise is not too rapid.

Sea level rise and land subsidence rates are reported in millimeters per year, where 1 mm/year= 0.39 inches/decade: RSLR means Relative Sea Level Rise, which includes any vertical movement of land, in our area subsidence. RSLR has been measured by tide gauges at some coastal sites for more than a century. Since 1993 it has also been measured by radar pulses reflected from the ocean surface by orbiting earth satellites. Land rise or fall rates are today being measured with GPS. Multi-annual fluctuations (winds, precipitation, etc.) create sea level ‘noise’ which must be smoothed out (filtered) to obtain a time series of sea level versus time. These data are then empirically fit with a simple algebraic function. This function can then be interpolated to get the relative sea level for any given time and place, and used to predict the near-term future.

Future climate change and sea level rise in Maryland have been professionally studied for decades. In 2007 the State established the MD Commission on Climate Change (MCCC), which includes a group of experts. The MCCC regularly issues reports, updating projections, and strategies for mitigation and adaptation. The most recent summary of MD sea level rise was issued in 2018. Projections have become more sophisticated, and are now couched in probability and statistics. For example, Baltimore, having Maryland’s longest tide gauge record, has a 66% probability of RSL being between 0.6 ft and 1.6 ft higher in 2050 than it was in 2000. RSLR has been accelerating at 0.084 mm/year per year—meaning +1.68 mm/year from 2000 to 2020! The prediction for Parker’s Creek would be very slightly higher.

Graphs of future sea level (and other parameters such as global temperature, CO₂, and human population) have the alarming, exponential-like shape of explosions. The curves resemble playground slides (if only 1900-2100 is shown), or hockey sticks (if the last 1,000 or 10,000 years are included). Eventually all such parameters will level off or/and decline. The question for humanity—and for the PCP– is what will happen in the interim.

Salt marshes like the one bordering tidal Parker’s Creek grow upwards via new but not decomposing organic matter from plant growth, plus any sediment washed or blown in. Marshes thrive when the RSLR is positive but slow. If RSLR increases too fast, some marshes will just migrate inland, leaving open water in their ‘wakes’. However this is possible only if the land is low and slopes are gentle. The Parker’s Creek marsh and swamp are bordered by relatively high and steep topography, thus preventing migration, i.e., a ‘wetland squeeze’. However, the hilly PCP topography also limits the amount of land flooded by future sea level rise, or land species killed by brackish water, or by shoreline erosion, were the marsh to become a lagoon.

Falling sea levels (caused mainly by ice sheets once again growing) would kill the salt marsh and turn the marsh-swamp of Parkers Creek into forest, somewhat as beaver ponds revert to forest once beavers are gone. However, sea levels won’t fall again for a very long time. Perhaps 15% of the atmospheric CO₂from fossil carbon fuels will stay in the atmosphere for millennia, enough to prevent glacial re-expansion.

What will happen in Parker’s is that the marsh may well not keep up with faster sea level rise, and would essentially drown by flooding. It’s probably not if but when! Scientists will study the process. Whether or not “we” should then artificially maintain the marsh is another matter– for ACLT land managers maybe not yet born.

Salt marshes have become more widespread globally in recent millennia, due to slowing of post-glacial sea level rise. As noted by paleoclimatologist Prof. William Ruddiman, ancient air bubbles trapped in polar ice sheets show that atmospheric methane (CH₄) and CO₂ began to increase again, respectively around 5000-6000 and 7000-8000 years ago. This did not happen during previous post-glacial times, leading Ruddiman to propose modest but significant pre-Industrial man-made greenhouse gas emission increases. The CO₂ increase corresponds to the time of rapid deforestation and farming expansion in Eurasia. The CH₄ increase corresponds to expansion of Chinese rice paddy cultivation. The global warming effects of these emissions happened to cancel out a natural 2^o C cooling and thus prevented some sea level fall. This implies that many modern salt marshes, including Parker’s, are ultimately due to mankind meddling with climate. (As it happens, Bill Ruddiman is a friend and former coworker, who in the early 70s coincidentally lived in Prince Frederick—in the Parkers Creek watershed).

US Geological Survey cores of Chesapeake Bay area salt marsh burial depths, age-dated by C14 (led by Dr. Tom Cronin of USGS), showed RSLR slowing around 6000 years ago, as the last of the Laurentide ice sheet melted away. This was more than five millennia after climate warming began. It takes time to melt a great ice sheet, and melting mostly occurs only in the warm season. There may have been further slowing to very low RSLR rates around 2000 years ago.

Sediment coring in Parkers marsh by a Johns Hopkins PhD student (Norman Froomer) in the 1970s showed the marsh layer is about 10 ft thick and 2000 years old at its base. The long-term average RSLR has thus been 1.5 mm/year, so the marsh kept up with this slow rise. (I suspect an open lagoon existed for some centuries before, but a barrier beach formed, leading to a marsh protected from Chesapeake waves).

New marsh data are being collected at present. In late 2019, the USGS (Dr. Michael Toomey) took three sediment cores in a short transect from the beach berm inland into the Parker’s marsh. This coring is part of a USGS study to see if marsh sediment piles record long-term storm surge frequency – registered by berm overwash sediment. The Covid-19 pandemic delayed analysis.

In the top 84 cm of the cores, inorganic sediment washed in from the Parker’s watershed records land clearing by early English farmers and their American descendants after the middle 17^th century. From this result, Froomer derived an average RSLR of 2.74 mm/year between 1650 and 1975. His other two study marshes, along the tidal Potomac, yielded the same result, but in Parker’s the transition from fresh tidal to brackish had migrated neither towards the Chesapeake (due to sediment input and increased runoff) nor inland (due to sea level rise and westward migration, by shoreline retreat, of the barrier beach). Those 10 ft thick marsh deposits are a priceless archive of past PCP watershed and marsh vegetation, historical land use, and sea level rise. Science has so far barely skimmed this archive.

Meanwhile CBL scientist Dr. Lora Harris and her Lab Assistant Jessica Flester have deployed (2012) four rSETs (rod-surface-elevation tables) in the marsh. The repeated measurements will determine how well the Parker’s salt marsh keeps up with RSLR. The data since 2013-14 offer good news. There is no tide gauge in Parker’s Creek, but Maryland tide gauge data reported in 2018 show RLSR rates since 2000 ranging from 4.2 to 7.3 mm/year. However, Parker’s Creek marsh elevations at the four rSET sites increased even more rapidly (+5.9-+13.9 mm/year), showing the Parker’s marsh (and others in our region) are so far doing well in keeping ahead of the sea level rise. Ironically, the main gas (CO₂) that is increasing global sea levels is also encouraging marsh plant growth.

Any change in living marsh vegetation over time and space can record the response to RSLR acceleration, which would increase both salinities and flooding frequency. For example, we predict the Parker’s salt marsh species like Spartina patens (salt hay) will be gradually displaced by the more salt tolerant S. alterniflora (smooth cordgrass). Upstream in the freshwater swamp, narrow-leaf cattail tolerates a bit more brackishness than the common broadleaf cattail. The May 16, 2020 drone image shows some dying trees in the small swamp forests—is this evidence for accelerated sea level rise and increased flooding, or/and increased brackishness? Or was this forest an ‘unnatural’ consequence of greatly increased sediment input from eroding farmland?

According to Dr. Patrick Megonigal of SERC (Smithsonian Environmental Research Center), recent research in the Kirkpatrick salt marsh shows that sedges tolerate more flooding and thrive more under higher CO₂ concentrations than do grasses like S. patens. Sedges use a different type of photosynthesis than grasses. Being familiar with the Parker’s marsh, Dr. Megonigal pointed out that our marsh vegetation is already dominated by types of plants like sedge, which should help stabilize our marsh against faster RSLR and extend marsh life by several decades.

How fast is sea level rising today? In recent decades relative sea levels have been rising around 3.5 mm/year in our part of the Chesapeake region, including the PCP. Of that, about 1 mm/year is due to land sinking, the remaining Earth response to the giant Laurentide ice sheet once north of here. Humans can’t change that. Of the remaining 2.5 mm/year, about half has been due to ocean warming and expansion, and the other half to glacier/ice sheet melting adding water to the ocean. Satellite sea level measurements beginning in 1993 plus data from ice sheets suggest that the effect of melting now dominates and has accelerated, with present RSLR likely more like 4.5 mm/year. Limited data from other marshes suggest that the Parker’s salt marsh may not keep up if this rate doubles.

An additional effect is possible Gulf Stream weakening, an additional RSLR effect in the Mid-Atlantic. The Gulf Stream circulation ‘holds’ a broad 3 ft high dome of water centered over the Bermuda region. This is the same Coriolis ‘force’ which also causes the counterclockwise circulation of surface Chesapeake Bay waters and thus keeps Parkers Creek tidewater less salty than across the Bay. If the Gulf Stream weakens, the dome height decreases, and water levels rise here, west of the Gulf Stream. For a salt marsh, even a few inches rapid sea level rise matters. Increasingly frequent ‘nuisance flooding’ (during normal high tides) in the Mid-Atlantic may in part reflect Gulf Stream weakening but is scarcely an issue for a nature preserve.

Long-term (multi-year) natural atmospheric fluctuations—reflected in temperature, winds, or precipitation, and unrelated to climate change—complicate identification of true climate change. One such natural multi-decadal oscillation has a quasi-period of 70-90 years and is generated within overturning of the Atlantic Ocean. This oscillation influences the atmosphere and is recorded as precipitation ‘cycles’ in bald cypress tree rings in the coastal Carolinas and by salinity-sensitive Chesapeake plankton sampled in sediment cores east of Parkers Creek (More watershed precipitation in the Bay watershed means lower Bay salinities). Whether climate change will affect this oscillation—which has been happening for millennia—remains unknown, but both the public and politicians may confuse such long term oscillations with climate change. As two possible local examples, the extreme 2002 drought might easily be blamed on climate change, but occurred 71-73 years after a worse drought (1929-31). In 2002 the freshwater tidal part of Parker’s Creek was brackish for weeks, and upstream of that, the creek turned to isolated mud puddles. How freshwater fish species survived or returned to Parker’s Creek remains a mystery. Some very cold Calvert winters during 1893-1905 were followed more than 80 years later by similar exceptional cold in the late 1970s-early 80s. Yes, I even ice skated on the Chesapeake Bay, Patuxent River, and Parkers Creek! Should we experience unusual cold winters in 2050-60, some might argue that climate warming has ended!

By far the most dramatic water level changes are short term storm surges caused by tropical cyclones-especially for storms tracking northeast with centers west of Parker’s Creek. The amplitude of such surges (roughly + 6 ft during Isabel in 2003) dwarfs the normal tidal range (about 1.46 ft at Parker’s Creek). Surge heights will be greater if Chesapeake shorelines become extensively armored. Future surges will at least add to the expected RSLR, and at worst be more severe if tropical cyclones are strengthened by the warming Atlantic. Future surges will carry brackish water up into parts of Parkers Creek presently upstream of the tides. Whether the PCP fully recovers from future extreme droughts, storm surges and widespread tree blowdowns will test its ecological resiliency.

Different future greenhouse gas emission scenarios give widely different model predictions (2017) of the 2100 increase over 2000 sea levels, from the most optimistic (+ 1 ft) to the most pessimistic (+8.2 ft). The +1 ft prediction is scarcely realizable because it assumed anthropogenic emissions would peak by 2020, which they have not, and that they will decline to zero by 2100. These are the mean global values, and sea level will likely be somewhat higher here. Our 1 mm/year land subsidence adds 4” per century, and any Gulf Stream weakening would add more inches. I will guesstimate +3 ft for the Parker’s Creek area between now and 2116. By 2373 a worst case might be up to +15 ft.

If RSL rises more than ca. ca 6-8 ft in the PCP area, the preserved PCP cliffs south of the Parker’s beach will slow their erosion and shoreline retreat rate (presently around 4”-8”/year) by Chesapeake’s waves, while the PCP cliffs north of the beach will start eroding faster. This gradual switch reflects 1) the tilt (dip) of the Miocene layers (strata) downwards to the southeast and 2) a more erodible layer, the Parkers Creek Bone Bed, currently exposed to wave action south of the beach, will disappear below the waves due to rising RSL. Meanwhile the same layer, currently above the reach of waves further north, will become accessible to wave erosion. Because the barrier beach protecting the salt marsh appears ‘pinned’ to -and thus aligned with- the cliffs, the slow clockwise rotation of the beach and cliffs will change to counterclockwise, and loss of salt marsh behind the beach will switch from greater in the south to greater in the north. These gradual changes are driven by the geology exposed in the cliffs but are speeded up by faster SLR.

Shoreline retreat along the PCP cliffs and the 1300 ft long barrier beach have not been specifically studied. The process can be envisioned as giant horizontal band saw cutting into the shore while gradually being raised. The beach and offshore sand bars migrate inland as they follow the saw. For an assumed ball-park average retreat of 3” per year, the PCP 7700 ft long shoreline will have migrated respectively 2.5’, 24’, 43’ and 88’ inland by the milestone years 1951, 2116, 2192, and 2373. By 2116 the Bay will therefore have taken about 4 acres of present PCP (including 0.7 acres salt marsh). By the year 2373 the corresponding land loss would be 16 acres total and 2.6 acres marsh. While sea level rise likely won’t much affect retreat rates, there might be little no shoreline retreat at all had it not been for increased anthropogenic greenhouse gas—but also, as suggested above, no Parker’s Creek marsh.

The sand budget of the Parker’s Creek barrier beach has not been studied. Because long-shore drift causes the beach to gain sand in the north and lose sand at the south, well-intentioned efforts to armor the cliffs and also reduce inland erosion north of the barrier beach may starve the beach. Storm waves would rapidly erode the salt marsh in the absence of the barrier beach. It is unknown whether high rainfall events allow Parker’s Creek to carry sand to the beach from tidewater limits, where the stream flows fast enough to have a sandy bottom. If it does, climate change, causing more frequent high-rainfall events, might help sustain the barrier beach.

RSLR predictions are closely related to greenhouse gas and therefrom temperature rise predictions. If mankind holds the rise to +2^oC, the 2116 RSL global mean may be only +1.5 ft, and the Parker’s marsh (+2 ft) may well survive. My +3 ft guesstimate for 2116 implies an average RSLR of around 9 mm/a, roughly twice the present rate. Will the Parkers marsh keep up? Maybe not—rates later during this century have to rise above 9 mm/year for that to be the average.

The spread of predictions reflects not only the largely geopolitical uncertainties of future greenhouse emissions, but also the response of two of the three major ice sheets—Greenland and West Antarctica. Each of these hides a ‘wild card.’ The current global CO₂ is around 415 parts per million, already the highest in at least the last 3 million years. If the average rate from now to 2100 is held near the present one (2.5 parts per million per year), the concentration would be around 600 ppm then. (900 ppm is a very pessimistic prediction which I think won’t happen: at least in the US, per capita greenhouse gas emissions have begun to fall).

USGS has sampled sediments from 3 million years ago and from 15 million year old Calvert Cliffs sediment—including from the PCP– to understand earlier warmer climates with higher greenhouse gas concentrations. However, there was much less ice on the planet during those warm times. Sea levels were much higher. There were no extinctions. However, we are now in uncharted climate territory—there is no known precedent for this much greenhouse gas and this much land ice at the same time.

Mass loss of the Greenland ice sheet has greatly accelerated in recent years, but this ice sheet is contained in a depression bordered by mountains. However even the mass loss so far has created a large pool of colder, fresher water (from admixed meltwater) south of Greenland. The climate cooling there is not good news, because the Gulf Stream system has a deep return flow. Turn off your water main and no water flows out of any faucet! If the cold ocean water is too fresh, it won’t sink, thus slowing or even stopping the return flow. A return flow shutdown apparently happened by rapid draining of a giant glacial meltwater lake from the Laurentide ice sheet 13,000 years ago, causing a 1200 year return to glacial climates in Europe and here. The climate here turned glacial in less than a century! Is the cold-water pool related to Gulf Stream weakening? A return to cold climates is unlikely because there is no large meltwater lake today, and because ice sheets can’t melt fast enough.Mass loss of the Greenland ice sheet has greatly accelerated in recent years, but this ice sheet is contained in a depression bordered by mountains. However even the mass loss so far has created a large pool of colder, fresher water (from admixed meltwater) south of Greenland. The climate cooling there is not good news, because the Gulf Stream system has a deep return flow. Turn off your water main and no water flows out of any faucet! If the cold ocean water is too fresh, it won’t sink, thus slowing or even stopping the return flow. A return flow shutdown apparently happened by rapid draining of a giant glacial meltwater lake from the Laurentide ice sheet 13,000 years ago, causing a 1200 year return to glacial climates in Europe and here. The climate here turned glacial in less than a century! Is the cold-water pool related to Gulf Stream weakening? A return to cold climates is unlikely because there is no large meltwater lake today, and because ice sheets can’t melt fast enough.

The West Antarctic ice sheet may have tipping points. Major ice streams like Thwaites Glacier that drain it are presently ‘pinned’ by grounding out on the continental shelf. Recently measured melting of ice sheet underbellies by warming sea water, coupled with faster surface melting, may cause this thick tongue of ice to lift from the sea bed. This might trigger rapid retreat by iceberg calving. Rapid collapse of the entire ice sheet may then be irreversible, something not yet captured in computer models. Some glaciologists studying the Thwaites suggest this tipping point may arrive as soon as 2060. By 2051 we will know much more.

The loss of polar ice sheet mass also slightly changes the Earth’s gravity field and thus the shape of the global ocean surface, i.e., sea level. Counterintuitively, the loss of Antarctic ice has more than twice the effect– increasing Maryland sea levels– compared to loss of Greenland ice.

We might take comfort in the resilience of the biosphere to dramatic climate- including sea level- oscillations especially in the last half million years. There have been few if any extinctions due to these changes. The megafauna (e.g., mammoths and sabre tooth cats) extinctions may be the only exception if rapid cooling 13,000 years ago was a cause, but other studies indicate the extinctions and climate change had the same cause).

As recently as 12,000 years ago, when people were already regionally present, the present PCP was forested with taiga conifers, and there was no Chesapeake Bay. Pollen studies suggest that it took only about a century for our present warm temperate hardwood forest ecosystem to migrate up here from the Gulf Coast once climates warmed around 11,300 years ago. Annual layers in Greenland ice cores tell us that this warming came with shocking speed but did not increase extinctions above background levels. The global warming only took about 40 years, in three different 5 -year spurts, probably driven by instabilities in ice sheets no longer present today. A five-year temperature anomaly today would not be recognized as climate change! The subsequent climate stabilization—until now– enabled the spread of agriculture and growth of civilizations.

The species responsible for our climate change and imminent coastal inundations may seem to be the biggest loser, but it’s also about many other biota that may now become extinct or further decline in population and range. Much of this is due to habitat loss-like deforestation and fragmentation- not caused by climate change. Rising temperatures, sea levels and CO₂ concentrations are only a part of how humans have stressed the biosphere. The future of ‘nature’ in the Parker’s Creek Preserve depends very much on what happens beyond the preserve—not only future climate change as discussed above, but also– to name just two examples– habitat destruction in the Latin American wintering grounds of migrant birds which nest in the PCP during summer, and the herbiciding of distant milkweed needed by migrating Monarch butterflies.

The ACLT-managed Parkers Creek Preserve can’t have any significant global impacts (except perhaps by land preservation and management examples). However, the PCP and especially the marsh is a priceless natural laboratory for studying how a relatively pristine Mid-Atlantic ecosystem naturally responds to climate change. This was a major reason for Maryland DNR support for PCP land preservation. The ACLT Science Committee and others have already made a great start in studying diverse components of this natural laboratory.

For helpful corrections and comments, I thank (in alphabetical order): Greg Bowen (Executive Director, ACLT), Denise Breitburg (SERC, ret.; now chair of ACLT science committee), Lora Harris (Chesapeake Biological Laboratory, CBL), Pat Megonigal (Smithsonian Environmental Research Laboratory, SERC), Bill Ruddiman (Univ. of Virginia, ret.), and Deb Willard (US Geological Survey)