The lithology of Great Valley and the
Valley/Ridge area consists of Cambrian and Ordovician limestones and dolostones
with interbedded sandstones and shales throughout The late Ordovician, Martinsburg Formation
acts as the cap rock for the karst system.
In adjacent areas of eastern West Virginia, you have the outcrop of
another karst aquifer called the Greenbrier Group, which contains a thick
sequence of carbonates and two important shale units. The late Mississippian-early Pennsylvanian,
Mauch Chunk Group acts as the cap rock for the Greenbrier aquifer. Stratigraphic columns are posted below.
Listed above are the two main karst aquifers in the study area of eastern Western Virginia and western part of Virginia. Here are some main facts about both aquifers.
Great Valley:
- In the Cambrian and Ordovician units the Beekmantown Group, which includes the Stonehenge Limestone, Rockdale Run Formation, and Pinesburg Station Dolomite are the most favorable units for karst units(Doctor et al., 2008). This is due to an increase in the purity of the limestones and dolostones in this unit.
- Karst can form in the Elbrook and the Concocheague Formations, but because those units are less pure karst features are not common as other units. The Concocheague consists of several beds of quartz sandstone and the Elbrook Formation is quite silty in nature. As a result these units are impermeable and can be quite resistant. These units can still feature karst areas especially purer limestones/dolostones of the Conococheague(Doctor et al., 2008).
- Karst development is also quite common in the New Market Limestone and the Chambersburg Limestone/Edinburg Formation.
- The shales of the base of Martinsburg Formation which is a sandy/shaly unit that acts as a confining unit/cap for the Great Valley Aquifer system.
Greenbrier Valley:
- Outside of the Great Valley is an adjacent area of the Valley and Ridge in which you have the Mississippian Greenbrier Group in eastern West Virginia. The Greenbrier Group is bounded by the Maccrady Shale which is the hydraulic base of the area, and the upper Mississippian/lower Pennsylvanian Mauch Chunk Group.
- The shales of the Greenbrier Group, especially the Greenville serve as important units for cave development. The Greenville Shale is quite resistant so it can act as a confining unit and a cap rock that caves can form underneath. Caves don't form in the Greenville Shale, but caves can form within shales of the Taggard Formation by way of fractures. Erosion process of streams can cause caves to form into the base of the Maccrady Shale (Palmer 2007; Palmer, 2009) The caves that form in the Greenbrier Valley are often called contact caves because they form at the contact between the shale and the limestone unit.
Great Valley:
- In the Cambrian and Ordovician units the Beekmantown Group, which includes the Stonehenge Limestone, Rockdale Run Formation, and Pinesburg Station Dolomite are the most favorable units for karst units(Doctor et al., 2008). This is due to an increase in the purity of the limestones and dolostones in this unit.
- Karst can form in the Elbrook and the Concocheague Formations, but because those units are less pure karst features are not common as other units. The Concocheague consists of several beds of quartz sandstone and the Elbrook Formation is quite silty in nature. As a result these units are impermeable and can be quite resistant. These units can still feature karst areas especially purer limestones/dolostones of the Conococheague(Doctor et al., 2008).
- Karst development is also quite common in the New Market Limestone and the Chambersburg Limestone/Edinburg Formation.
- The shales of the base of Martinsburg Formation which is a sandy/shaly unit that acts as a confining unit/cap for the Great Valley Aquifer system.
Greenbrier Valley:
- Outside of the Great Valley is an adjacent area of the Valley and Ridge in which you have the Mississippian Greenbrier Group in eastern West Virginia. The Greenbrier Group is bounded by the Maccrady Shale which is the hydraulic base of the area, and the upper Mississippian/lower Pennsylvanian Mauch Chunk Group.
- The shales of the Greenbrier Group, especially the Greenville serve as important units for cave development. The Greenville Shale is quite resistant so it can act as a confining unit and a cap rock that caves can form underneath. Caves don't form in the Greenville Shale, but caves can form within shales of the Taggard Formation by way of fractures. Erosion process of streams can cause caves to form into the base of the Maccrady Shale (Palmer 2007; Palmer, 2009) The caves that form in the Greenbrier Valley are often called contact caves because they form at the contact between the shale and the limestone unit.