Nature Nearby: The not-so-solid ground beneath our feet
BY FRED SEITZ
As I take my usual neighborhood walks, I’ve been slipping and sliding on our muddy soil. While the gravel scattered on some pathways provides some traction and a seeming sense of solidity, we cannot forget that our homes are on the western portion of the Atlantic Coastal Plain — an area that was under the drowned valleys of the Susquehanna River about 35 million years ago.
Today, we refer to a remnant of that river valley as the Chesapeake Bay. Our neighbor here in Hyattsville, the Anacostia River, is the largest tributary to the Potomac River; the Potomac, in turn, feeds into the Bay.
Water. Soil. Gravel. The ingredients in the slippery ground under our feet may seem mundane, but they tell ancient and far-ranging stories.
The geologic history of our coastal plain involves ice ages, ocean processes, millennia’s worth of deposits from river runoff — and even the heavens.
Let’s start with the heavens. About 35 million years ago, an asteroid or comet of an astounding size (larger than a mile in diameter!), traveling at about 144,000 mph, crashed near what is now Cape Charles, Va. The impact resulted in a 12-mile wide crater. (The Chesapeake Bay Crater is the largest such formation in the U.S. but was only discovered in 1990 because it’s beneath the floor of the bay, under some 1,000 feet of rock.)
Millions of years later, the river that we now know as the Susquehanna carved a canyon as it flowed from the Appalachian Mountains to the Atlantic Ocean. And about 10,000-20,000 years ago, the end of the most recent ice age triggered rising sea levels, which flooded the Susquehanna River valley. Over millennia, runoff from mountains and hills has filled that channel with sentiment and has transformed the broad mouth of the river into the large, shallow region that we now know as the Chesapeake Bay.
The current ground beneath my slipping feet is mostly clay, which is a very fine-grained and dense material.
While clay has been used by humans for thousands of years to make pottery, its structure was not well understood until the 1930s, following the development of X-ray diffraction techniques and improvements in microscopic and thermal analyses. While clay usually contains some combination of silica, aluminum, magnesium and iron, this mix of minerals only becomes true clay with the addition of water.
In drier times, the clay in my yard has hardened, making digging and planting extremely challenging. Despite the frustration that results, I remind myself that this propensity to harden is why we use clay for ceramics.
Over the last few months, however, we’ve often had plenty of water to keep the clay underfoot soft — and sometimes even treacherous. On occasion, extremely saturated clay has partially given way under me and darn near swallowed my foot.
Humans have added pebbles and gravel to clay in an attempt to provide more traction and stability to our local trails. We likely have harvested these pebbles from a variety of waterways, and they are the result of the consolidation of sand, local soils and other materials with subsequent polishing by rivers and streams.
So, the shifting ground beneath my feet has an impressive and complex geologic history and structure. It has been formed over millions of years by cosmic forces, shifting oceans and rivers, glacial melts, sedimentation — and, of course, terraforming by humans. It’s only fair if our world-shaping efforts sometimes strike back and remind me of my place (or lack thereof).