Maryland’s Geologic Regions
America in Miniature
One of Maryland’s nicknames is “America in Miniature” due to the wide variety of terrain within its 10,460 square miles. To drive across the state is to witness dramatic changes—from the flat salt marshes of the Eastern Shore, to the urbanized landscapes of Baltimore, to the towering forests of Western Maryland. But what drivers can’t see are the equally dramatic differences beneath these landscapes and how they filter, hold, and release groundwater stored there.
Here’s a look at the state’s five distinct geologic regions:
COASTAL PLAIN
Size
Covers 50 percent of the state, approximately 5,000 square miles
Characteristics
- Flat, with loosely arranged sediments of gravel, silt, and clay, it is the youngest geological formation in the state at about 144 million years old.
- The Chesapeake Bay bisects it, creating one groundwater region separated by America’s largest estuary.
- Sediment layers become thicker moving east from the Baltimore region toward Ocean City.
- Soils on the Eastern Shore are sandy and permeable, and the landscape is flat. Drainage is poor because of the high water table and shallow stream incision.
Groundwater concerns
Most residents of the Coastal Plain get their drinking water from groundwater. Saltwater intrusion is a threat and may require some residents to drill deeper into the ground to find fresh water.
Some farmers are also withdrawing more water to irrigate fields because of inconsistent weather patterns, causing scientists to worry about quantity.
PIEDMONT
Size
Covers 25 percent of the state, or approximately 2,500 square miles
Characteristics
- The Piedmont consists of more solid rock than the sandy and more permeable Coastal Plain sediments.
- Groundwater aquifers occur within fractures in the Piedmont rocks.
- There is an abrupt change in elevation between the Coastal Plain and Piedmont region, which has steeper topography.
- The Piedmont region’s elevation ranges from an average of 350 feet in the Frederick Valley to more than 1,200 feet at Sugarloaf Mountain.
- Many Piedmont residents rely on reservoirs for water supply. Farmers, however, use both groundwater and surface water sources for irrigation because groundwater supplies alone are not sufficient.
Groundwater concerns
The Piedmont relies on rainfall to replenish its surface waters and its aquifers, and precipitation has been inconsistent over the last several years, causing cycles of drought. In Frederick County, Maryland, as in other areas facing urbanization, rapid growth can result in streams being channeled into pipes or paved over with concrete and asphalt. Such changes can limit the ability of streamwater to soak into the ground and recharge groundwater, according to Andrew Elmore, an ecologist at the University of Maryland Center for Environmental Science.
BLUE RIDGE
Size
Covers about 5 percent of the state, approximately 500 square miles
Characteristics
- The Blue Ridge is a small part of Maryland, but it underlays many other states, including Pennsylvania.
- Rock here, much of it hardened sedimentary and metamorphic, is more resistant to erosion than other formations in Maryland.
- The area includes three mountain ridges—Catoctin Mountain, South Mountain, and Elk Ridge—all composed of quartzite, a very resistant rock.
Groundwater concerns
Due to mountainous terrain, there is not much worry concerning growth and groundwater shortages, as the region accounts for only one percent of the state’s groundwater use.
RIDGE AND VALLEY
Size
Covers about 10 percent of the land, approximately 1,000 square miles
Characteristics
- The region is composed of erosion-resistant shale and sandstone (ridges), and broad, flat valleys of weaker layers of limestone.
- As with the Blue Ridge, this region comprises a small part of Maryland, but its parallel ridges and intervening valleys extend through parts of several other states.
- Due to the area’s limestone deposits, the region includes soils, known as the Hagerstown series, that the U.S. Department of Agriculture declared among the best in the nation for growing certain crops.
Groundwater concerns
Groundwater wells are most commonly drilled into the limestone rock aquifers of the region. However, because these rocks commonly have small, naturally occurring channels, these aquifers can be more vulnerable to contaminants such as fertilizers introduced at the land surface. They also can have sinkholes, which can foster even faster movement of contaminants through the near-surface aquifer. Groundwater used for public water supply typically come from aquifers that can be over 250 feet beneath the surface and are less susceptible to activity on the land’s surface.
APPALACHIAN PLATEAUS
Size
Covers about 10 percent of the land, approximately 1,000 square miles
Characteristics
- This region is Maryland’s coal country, consisting of hardened shale, siltstone, and sandstone, with layers of coal preserved in this formation.
- Groundwater occurs in fractured rock similar to the other formations in Maryland, with the exception of the Coastal Plain.
- Public water supplies usually come from both surface and groundwater sources, Most rural areas use private wells for water supply.
- This area, only partially situated in the Chesapeake Bay watershed, is home to Deep Creek Lake, Maryland’s largest freshwater lake.
Groundwater concerns
Arsenic was detected in a small percentage of wells at levels exceeding the U.S. Environmental Protection Agency's drinking water standard for maximum contaminant levels.
Graphics by Lisa D. Tossey / MDSG, redrawn from USGS graphic
