A few weeks ago, I had the good fortune to accompany the Smithsonian Paleobiology department on a field trip to Theodore Roosevelt Island, a tiny bit of land in the middle of the Potomac river, between Washington DC and Virginia. The island represents a neat little microcosm of history in our nation's capital, from the time when it was housed a populous Nacotchtank fishing village (first recorded by Capt. John Smith as he sailed up the Potomac), to the days when it was cleared for the manor of John Mason (son of George Mason, the early American statesman), to the current day where it boasts a granite monument for Teddy Roosevelt. Today, most of the island's 80-odd acres are covered with forest, wetlands, and rocky outcrops.
For students of geology, the island is a great place to observe many distinctive regional features and landscape morphologies. Perhaps the most significant is the "Fall line" that bisects the island- an unconformity where hard bedrock (schist and gneiss of the Piedmont Province) stands in relief against an eroded plain of soft sediments (called the Coastal Plains Province.) This is a local expression of a much larger continental feature-the 900-mile Atlantic Seaboard Fall Line that runs from New Jersey to Georgia. On the island it is not much more than a low slope, but it serves as a great illustration of the geology, unobstructed by buildings or roads. .
The image above shows how the bedrock of the Piedmont slopes down under the Tertiary and Cretaceous sediments that constitute the Mid-Atlantic coastal plain. DC, like Theodore Roosevelt Island, is neatly divided by this line, with northwestern neighborhoods like Georgetown, Adams Morgan, and Cathedral Heights perched high on the Piedmont and the rest (including the National Mall) lying low on the Coastal Plains.
Through out much of DC, the visible Piedmont bedrock belongs to the Sykesville Formation. On the island it appears as schist (a rock composed of metamorphosed ancient mud layers) rich in garnet and biotite mica. The garnets represent inclusions that were especially rich in iron, magnesium and aluminum.
Scattered among the native bedrock there are some river-rounded stones brought in for filling gravel paths. On one of these, we were lucky enough to find the fossil trace of a prehistoric burrowing worm called Skolithos.
Another interesting feature is the presence of ubiquitous shoreline structures like natural levees and backswamps (see below, with the office buildings of Rosslyn in the background.) As sediment washes up on shore it piles up and is anchored by tree roots, building a low, long mound along the shore. Underground seepage, however, allows river water to form marshy "backswamps" where water-logged conditions prevent the growth of trees.
It is astounding how geologically rich such a small piece of land can be. And while Theodore Roosevelt Island is certainly special, it is not necessarily unique- there are many pockets of geological knowledge to be discovered all around Washington DC.
Thanks to Dr. Ray Rye, leader of the expedition.
Further Reading:
Means, John (2010). Roadside Geology of Maryland, Delaware, and Washington DC. Missoula: Mountain Press Publishing Company.
