Bay’s coastal cliffs are but shells of their former selves
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Weather significantly troubled Capt. John Smith and his crew in early June 1608 as they explored Chesapeake Bay's Eastern Shore. Thunderstorms had torn their shallop's sails, broken a mast and trapped them against Bloodsworth Island for days. Frustrated by winds that had opposed their sailing through a nearby channel, they called it the Straits of Limbo.
Eventually, a fair wind and tide let them sail through into the mainstem Chesapeake on June 11, 1608. As they came out of what is today called Hooper Straits, they saw the Bay's distant Western Shore for the first time. Scanning south to north, amid a dark line of distant trees swimming in mirage against the horizon, they saw a bright flash of sunlight on open sand and clay cliffs. These drew both the eye and interest, and Smith shaped his course across the Bay toward these "Mountaines on the [mainland]."
Smith named these "Rickard's Cliffes" after his mother's English family. Calvert Cliffs, as they are known today, provide a cross section into 20 million years of Chesapeake history.
This cliff stood out in Smith's memory as he later recounted his exploration of the Bay, but there are other cliffs with their own histories along margins of Virginia's river necks, the southern shore of the Potomac at Nomini and on its northern shore downstream of Popes Creek. There are also cliffs on the Patuxent western shore at Drumcliff and on the Bay's upper Eastern Shore around Worton and Turkey Points.
Over the next few hundred years, geologists unraveled the complex sedimentary record in these cliffs, which extends back many millions of years before Chesapeake Bay was created. Our overview on the sequence of these events, the former muddy basins, sandy shallows, vast shell beds and the remains of sharks and whales, has been continuously refreshed by shoreline erosion-between 100 to 400 feet-over the subsequent four centuries.
With each collapse of a cliff face in winter breakdowns and with each sorting of beach sediments by wave action at the toe of the cliff, fossil remains of creatures from these ancient habitats are revealed. Geologist Peter Vogt calls this continuous erosion process "the windshield wiper on Maryland's past."
Their wide panoramic views make these cliffs tempting home sites. But once erosion events occur along unstable banks, there is immediate public pressure to stabilize and harden these shorelines. Techniques have included bulkheading, stone revetments, groins perpendicular to the shoreline, kudzu plantings and offshore breakwater structures to retard the erosive effects of waves. The result, Vogt, points out, is the eventual turning off of the windshield wiper that has revealed the Chesapeake's history.
Drumcliff on the Patuxent, for example, is a "type locality," a place where a geological feature or particular fossil species was first discovered. Homeowners, fearing for the loss of their property, have stabilized the shoreline with rock. Formerly, scientists could look repeatedly at a site like this for clues about life stages, development of descriptive characters and the evolution of an organism now extinct. With thousands of species rapidly facing extinction today, understanding the fate of past inhabitants, without the effects human influence, can be useful.
In 1998, I attended a field trip with the American Malacological Society at a privately owned site just north of Long Beach called Matoaka-which incidentally, was Pocohontas' actual Indian name. Fossil collectors from the Smithsonian, Academy of Natural Sciences of Philadelphia and Europe have come to these cliffs for more than a century to find specimens from hundreds of extinct species. The host swarmed over the cliffs, plucking out examples of the beautiful Ecphora quadricostata, an extinct, four-ribbed and amber-colored snail, some big as a fist. It was selected years ago as Maryland's state fossil.
We walked along exposed horizontal beds containing millions of little spiraling Turritella snails. These shells were likely dead when they accumulated in these deposits, washed there by waves or currents, on some sea floor depression or along a shoreline-much like the shell "hash" swimmers find at the undertow on many modern beaches.
At other sites, there were beds of the huge Miocene oyster Isognomon. Large quantities of these shells formed layers. When they came into contact with groundwater, these layers dissolved, re-precipitated and solidified into limestone conglomerate. Elsewhere, these beds form rock shelves, even caves where exposed on the shoreline. They are natural beach stabilizers, preventing further erosion over a human lifetime.
These shellfish had an incredibly thick shell, with 1.5-inch hinges joining the individual valves of a mollusk. Had they not become extinct, a few of these would have provided a substantial human meal. Fossil pearls have even been found in some.
From a vantage 20 feet up a 100-foot cliff, I tried to imagine what this ancient shore might have been like 15 million years ago. One thing I did know: It was not a scene like today's Chesapeake.
Geologist and paleontologist Ralph Eshelman said that from roughly 23 million to 5 million years ago, during the Miocene Epoch, this had been what scientists now call the Salisbury Embayment, a bight or cove-hundreds of miles wide-along the edge of an ancient sea. A cape to the south of the modern Chesapeake and a long curved seacoast bowed westward toward the Appalachians, arcing northeastward to a second cape where New Jersey's Pine Barrens lie above sea level today.
The fossil record at Calvert Cliffs indicates that the climate might have been similar to that of the coast of Georgia. It was a feeding and calving ground for many extinct marine mammal species. One can't walk very far along the shoreline beneath today's towering cliffs without finding a fragment of bone from one of these species. Very often, whole skeletons emerge from the cliffs. Paleontologist Steven Godfrey and colleagues from the Calvert Marine Museum, acting on a tip from an amateur fossil collector, excavated much of an extinct whale, including the skull which, when packaged in protective plaster, was so heavy a Navy helicopter from the Patuxent Naval Air Station was engaged to hoist it for conservation at the museum.
I imagined a pod of these ancient cetaceans, steaming across the ancient, shallow Calvert Sea. Some were baleen whales, which filtered tiny organisms from concentrated plankton blooms in the sea; others were toothed whales, predators of abundant, smaller species.
The stratigraphy of this area was laid down long before the birth of today's Chesapeake and the several similar coastal estuaries that preceded it as sea level rose and fell with the passage of time. The present Bay has incised through and eroded modern shorelines to expose deep history. Beneath the Pleistocene beds, which cap the cliffs, are the exposed layers of increasingly older Eastover, St. Mary's, Choptank and Calvert formations.
Another participant on the field trip, Lauck "Buck" Ward, then curator of paleontology at the Virginia Museum of Natural History in Martinsburg, said that the beginning of the 50-million-year-old Nanjemoy formation lies just below surface deposits in Northern Virginia.
The surface upon which rainfall lands is close enough to ground level here that downward percolation recharges or replenishes an underground reservoir of water bearing this name. This is an aquifer-a layer of permeable sands and gravels, confined between layers of water-impermeable clays, which slopes downward toward the East and South, dipping hundreds of feet beneath the ground. Along and beneath the Chesapeake in southern Maryland, 50-odd miles away, the aquifer is the source of water for many municipal and drinking water wells. It is frightening to look at the actual recharge fields for the Nanjemoy and other aquifers of the region as they are now covered with impervious surfaces and leached full of the contaminants of human development.
Ward pointed out the Aquia formation, which is 60 million years old, in a lower layer of sediments. It is another aquifer, also feeding modern wells at different locations.
My wife, Nancy, and I were intrigued by his knowledge of the basin's Coastal Plain and Piedmont, and Ward agreed, to meet with us later that year, in mid-November, for a tour back in time. We met him with his skiff at Fairview Beach on the Virginia Potomac, and took off.
With the strata of ancient sediments tilting downward toward the modern ocean, material of much greater age is exposed here than at Calvert Cliffs. We nosed ashore, literally landing on what was once the floor of the shallow Paleocene sea roughly 65 million to 56 million years ago. We waded across a pavement of much larger Turritella snails-the distant ancestors of those seen on the Bay front, these up to 5 inches long. There were also different, very large oyster fossils. Some individual shells were 8-by-9 inches, and grew lying flat on their backs. Ward said that the frilled edges of the oyster's rapidly expanding bill-where the shellfish's living mantle was generating new shell-could be as wide as an inch. These protruding foliose processes helped to prevent the shellfish from sinking and being smothered in softer, alongshore sediments.
Farther up the river, we came in to shore again. Beyond the next Point was Aquia Creek, where historians believe Smith, during his 1608 explorations, sought out the Indian mine of "spangled scurf" which he derided, but the councillors at Jamestown sent back to England, hoping-in vain-to find some precious metal.
Here, on the beach downstream of Aquia, we found many sharks' teeth, symbolic of an era rich with life that supported top predators. This must have been a coastal area, because we found evidence of characteristically "warty" crocodile bones, and strangely, the fossilized coprolites (feces) of these reptilian predators preserved through millions of years in ancient muds.
This was all 65 million to 56 million years ago. Ward said that much of this ancient world survived the great Cretaceous-Tertiary extinction event within this period that suddenly ended the age of dinosaurs. There was yet another event, he said, perhaps 50 million years ago which pounded scores of species out of existence.
I thought this must have been the bolide-likely a comet or asteroid mass-which struck the lower Chesapeake near Exmore on what eventually became Virginia's Lower Eastern Shore about 53 million years ago. It was an astounding ecological disaster, hurling immense bomblike missiles outward as far as the Patuxent, each of which deeply cratered the earth, and created tsunamis of debris and water. Ward said that after this event, there appeared to be only 15 or 16 species of mollusk remaining in this geographic region from the whole fauna.
Next, we drove 50 miles to the Doswell Quarries being excavated by Martin Marietta Corp. Here in what is an otherwise flat and sandy rural landscape was a great pit. I estimated that it was 1,000 feet long, 500 feet wide and 100-200 feet deep; perhaps 8.5 million cubic yards of schist had come out of there.
Schist is not easily dated. It's a metamorphosed rock-melted and mixed like pudding beyond recognition of its Precambrian origins. We've all seen this crushed stone before, as a product known as "aggregate," blended into cement. It is the stuff of foundations, bridge piers and abutments all around the Bay and its cities.
Millions of years ago, this rock was a hump in the sea floor of the Salisbury Embayment. Ward swept his arm south and west, indicating where this embayment lay, rich in those ancient times with marine life. He showed us the remains of a sperm whale that had been found nearby, buried in the former shallow seabed which covered the rock being mined today. Its jaw had protruded from a nearby embankment. He picked up part of a tooth: "If we marked off that 100-by-150-foot site, we might have 10 whales in there."
And their predators: Ward used a geological pick to scrape away years of time at the contact between the interface of both the Aquia and Nanjemoy formations. Worm or crustacean burrows from one bed cut into the sediments of the other, millions of years greater in age. All of these marine materials lie atop, and in the way of mining the hard rock beneath. Ward looked at his excavation: "I don't think I've ever found one of these so large, he said, handing Nancy a 3-inch shark tooth. "This must have been a 20 foot shark."
These were ancestors of the Chesapeake's immense signature prehistoric shark Charcarodon megalodon. Their fossils, Ward said, begin here in the Aquia, and are distinguished by side or lateral cusps (little sub-teeth on each side of the main point). The teeth increased in size and the cusps decreased as they evolved over time. Some of the Miocene sharks might have reached 43 feet in length. No such creatures survive today.
The physical makeup of these fossils, almost crystalline in appearance, is unusual and seems less perfectly mineralized than the younger Miocene ones we are used to along the Bay. My own scrapings at the Doswell site revealed tiny, shark teeth weathered by rain and sunlight that easily crumbled in my fingers.
Black stones in the deposit, Ward said, are phosphate, and these marl-like deposits-marl is rich in carbonate of lime-might well have been the "plaistering gypsum" that Thomas Jefferson, a forward thinking farmer the early 19th century, believed would restore Virginia's played-out agricultural soils. Marls elsewhere in the Chesapeake were among the first mineral fertilizers colonial farmers applied to their soils.
What interesting links weave together among the deep past, historic times and the modern Chesapeake.
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