Point 1: Why Are Calvert’s Cliffs Exposed?
The Miocene epoch of geology lasted from 23 to 5.3 million years ago. The middle Miocene was a time of high sea levels worldwide. The fact that we have these marine sediments exposed today, above present sea level, partly reflects that sea levels are generally down from what they were.
Shallow seas, embayments of the Atlantic, episodically covered much of Southern Maryland from about 18 to 20 and eight to 10 million years ago. Sea level rose and fell, probably because ice volume in east Antarctica grew and shrank on about million-year intervals.
The great Calvert Cliffs exposures — and some equally high but shorter cliffs along the Potomac (Nomini Cliffs, Virginia) — are ultimately the result of shoreline erosion by the Chesapeake Bay and its tidal tributaries.
I often hear that the cliffs are millions of years old. Untrue! The cliffs are forming now. They are present features of the topography, as opposed to the old sediments exposed in the cliffs. The same could be said of most all our erosional landmarks, for example the Grand Canyon.
If active shoreline erosion were to cease (that is, if climates were to cool, causing sea levels to stabilize and then fall), the cliffs would disappear, turning into forested slopes, about 30 to 45 degrees. I tell students that erosion is the natural windshield wiper that keeps our window into Maryland’s ancient past clear.
Of course, folks with real estate on or near the cliffs have less charitable views of shoreline erosion.
Point 2: How Our Creeks, Rivers and Bays Were Formed
After the Miocene seas withdrew to the east, our region was low, gently sloping, dry land. This pattern persisted from about eight to one million years ago. The Potomac then flowed more or less straight out to the sea from the area of Washington, D.C.
Perhaps one million years ago, for reasons not well understood, the Potomac shifted to its present southerly course out of the Washington area. The Patuxent River developed, as did tributaries draining into it from east and west.
Among these many tributaries was one that began on uplands now under Chesapeake Bay. This flowed by way of present Parkers Creek (opposite to the present flow direction!) and Battle Creek into the Patuxent.
Then Parkers Creek was pirated by the Bay from the Patuxent and inverted. The inversion took place over a period of time, possibly beginning around 200,000 years ago. A short piece of the original Battle-Parker Creek is preserved as a dry valley crossed by German Chapel Road in Calvert County. The elevation is about 40 feet above sea level, so we know the stream flowed at that elevation at that point before the piracy.
What accounts for the stream piracy? Ultimately the cause is Calvert Cliffs’ erosion by earlier Chesapeake Bays.
A sequence of Chesapeake Bays formed, one during each warm period, by tidewaters flooding the lower reaches of the ancient Susquehanna. During these intervening glacial periods, the Susquehanna carved channels, all presently about 200 feet below sea level and all but the latest — now the shipping channel — long filled with younger sediment.
Every new Chesapeake Bay eroded more of the uplands, forming earlier Calvert Cliffs. At some point, an earlier Chesapeake chewed into the headwaters of the original Battle-Parker Creek. Eventually the new east-flowing stream, young Parkers Creek, eroded its way back to the middle of Calvert County.
So, things have stabilized, but the Chesapeake continues to gnaw away land, most rapidly along the Calvert Cliffs. Instead of the tidewaters gradually withdrawing due to climate cooling that would otherwise be occurring, anthropogenic greenhouse gas emissions will make our sea level continue to rise and shorelines erode for centuries.