Shaped Over Millions of Years
Southern Maryland Geologic History
Story by Edna Troiano | Photography by August Selckmann | Reprinted from Summer 2015
Early Southern Maryland history conjures up images of the landing of the Ark and the Dove, or perhaps of the tribes thriving in the land prior to the colonists’ arrival. But mil-lions of years before humans inhabited the region, crucial events shaped the land and created an environment rich for farming, fishing and settlements. Here’s the countdown.
ABOUT 175 MILLION YEARS AGO
Southern Maryland was originally part of a supercontinent called Pangea (Greek for “all-earth”). As the plates that glide over the mantle (the layer that covers Earth’s core) began to shift, what is now the west coast of Africa broke away from North America. As Pangea broke apart and drifted, separate continents gradually formed.
ABOUT 35 MILLION YEARS AGO
In 1983, a drilling core near Atlantic City revealed a geologic surprise: something – probably an asteroid or comet –had crashed into Earth near the tip of the Delmarva Peninsula, creating a huge impact crater. Originally thought to be about 24 miles wide and nearly as deep as the Grand Canyon, researchers for the National Geographic Society now speculate that the bedrock may have been shat-tered to a depth of seven miles and the width of the crater may have reached 85 miles. The crater is the largest in the con-tinental U.S.. The impact propelled water and rock miles into the air and caused a tsunami that not only swept over Southern Maryland, but may have even topped the Blue Ridge Mountains.
ABOUT 24 TO 5 MILLION YEARS AGO
During the Miocene epoch, which stretched over 20 million years, sea levels fluctuated. Each time the warm, shallow sea rose over the Atlantic Coastal Plain, which includes Southern Maryland, water deposited shells, bones and debris; each time it receded, it left a layer of silt, sand, gravel and clay, covering the shells.
Calvert Cliffs, which runs for 24 miles along the Chesapeake Bay, is noted for the fossils embedded in the cliffs. Peter Vogt, a marine geophysicist formerly with the Naval Oceanographic Office and the Naval Research Laboratory, compares the cliffs to a layer cake with alternating tiers of fossils and sediment. Looking at the cliffs, he says, is like look-ing through a window into what underlies most of Southern Maryland. The fossils within the Cliffs are millions of years old. Although most fossil hunters seek shells and sharks’ teeth –including those of the giant prehistoric megaladon – over 600 species of fossils have been identified. Vogt identifies Pope’s Creek in Charles County as another fossil-rich area.
Fossil hunters frequently ask if they can find dinosaur bones. Vogt says that dinosaurs had been extinct for about 50 million years when the seas covered Southern Maryland. However, sediment from the age of dinosaurs lies less than a thousand feet under southern Calvert and St. Mary’s counties.
ABOUT 110,000 TO 12,000 YEARS AGO
During the last glacial period, commonly (but erroneously) called the last Ice Age, a sheet of ice a mile thick extended as far south as Pennsylvania. Although the glacier didn’t reach Maryland, it had an enormous effect. As the glacier melted, the sea rose, and water carved channels that created streams and rivers.
The rising water covered the lower Susquehanna River Valley, helping eventually to create the Chesapeake Bay. The Susquehanna River Valley still exists as the deep channel within the Bay.
Toward the end of the Ice Age, the first humans, presumably hunters and gatherers, arrived. Although they may have come as early as 18,000 years ago, archeologists have found tools and points confirming that the area was inhabited by 11,500 years ago.
THE YEAR 2015
The land and waterways we take for granted formed over millions of years. As Vogt says, if we look at Southern Maryland as a sculpture, then water was the sculptor. But the water and land are always in flux. Erosion causes Calvert Cliffs to change daily. And rising seas will impact the Bay, rivers and land in the future. Geologic change – although slow –will remain constant.
To learn more about the geology of South-ern Maryland and to view fossils, visit the Calvert Marine Museum on Solomons Island (www.calvertmarinemuseum.com). The muse-um is open daily from 10 a.m. to 5 p.m. Visit www.mgs.md.gov/geology/fossils/fossil_collect-ing.html for details on fossil-collecting sites.