Structural Influences that shape the Valley and Ridge and their Influences of Cave Passage Morphology
The Valley and Ridge has undergone a rifting event and three mountain building events (Taconic, Acadian, and Alleghanian). Due to these events the crust was deformed creating a complicated arrangement of folds and faults. Each of these features are critical in the formation of the karst landscape in this area.
Folds:
-Both axial planar cleavage and jointing can be zones of enhanced dissolution.
-The nose of synclines and anticlines are very prone to the development of sinkholes. Also the tightness of the folding and the resultant lithology is important to note for the development of a depression (Doctor et al., 2008).
- The Elbrook Formation which is a slity dolomite for instance favors more spring development and not sinkholes, but more purer carbonates like the Beekmantown Group favor depressions, especially at the noses of the folds (Doctor et al.,2008 )
- Cave development is heavily influenced by folds, as shown in the picture below. For example, Luray Caverns, Crystal Caverns, and Grand Caverns form in the noses of folds (Doctor et al.,2008)
Also cave development in the Great Valley is common where a bedding plane, joint, and a fold axis all meet.
-Both axial planar cleavage and jointing can be zones of enhanced dissolution.
-The nose of synclines and anticlines are very prone to the development of sinkholes. Also the tightness of the folding and the resultant lithology is important to note for the development of a depression (Doctor et al., 2008).
- The Elbrook Formation which is a slity dolomite for instance favors more spring development and not sinkholes, but more purer carbonates like the Beekmantown Group favor depressions, especially at the noses of the folds (Doctor et al.,2008 )
- Cave development is heavily influenced by folds, as shown in the picture below. For example, Luray Caverns, Crystal Caverns, and Grand Caverns form in the noses of folds (Doctor et al.,2008)
Also cave development in the Great Valley is common where a bedding plane, joint, and a fold axis all meet.
Faults:
-Faults contribute greatly to the karst hydrology of the Ridge and Valley Province. Common faults are thrust faults that were formed during the orogeny events these include both thrust faults and normal faults. The strike-slip(cross-strike) and oblique faults tend to be excellent pathways for groundwater motion in the Great Valley Karst (Kozar and Weary, 2009).
-Work at Opequon Creek area shows that 53% of all the springs occur within 300 meters from a fault system, with 80% of springs that contain a discharge of 1,000 gallons/minute also located within that 300 meter zone of a major fault (Kozar and Weary, 2009). In the Greenbrier Group, the bedding is not as deformed compared to the Valley and Ridge, but still the strata is folded gently (Palmer, 2009).
-Faults contribute greatly to the karst hydrology of the Ridge and Valley Province. Common faults are thrust faults that were formed during the orogeny events these include both thrust faults and normal faults. The strike-slip(cross-strike) and oblique faults tend to be excellent pathways for groundwater motion in the Great Valley Karst (Kozar and Weary, 2009).
-Work at Opequon Creek area shows that 53% of all the springs occur within 300 meters from a fault system, with 80% of springs that contain a discharge of 1,000 gallons/minute also located within that 300 meter zone of a major fault (Kozar and Weary, 2009). In the Greenbrier Group, the bedding is not as deformed compared to the Valley and Ridge, but still the strata is folded gently (Palmer, 2009).
Joints/Bedding Planes: Joints and to a lesser extent Bedding Planes have an important control to conduit development. The intersection of joints with bedding planes is important hydrologically in the Great Valley (Orndorff and Harlow, Jr., 2002). The Greenbrier Valley, the caves take on more of a bedding plane control as evident by the sinuosity of the passageways, but can still be influenced by structural features as shown below.
Strike/Dip: The strike and dip can dictate the direction of cave development. In the Great Valley, caves passages tend to follow the strike of the limestone/dolostone beds (Palmer, 2007; Palmer, 2009). Also the steeply dipping rocks provide a very steep gradient for the groundwater to flow down gradient and in addition to the faults and lithological contacts, leads to spring formations. Also sinkholes seem to be concentrated in zones of faulting in the Great Valley (Doctor et al., 2008). In the Greenbrier a moderate strike and dip occurs.
Gradient: In the Great Valley the gradient is influenced by both structural and lithological variables. The range of elevation is quite significant in the Ridge and Valley and this is related to structure and the permeability of the rock. The upload areas are formed by the more resistant units such as the Conococheague and Martinsburg Formation. You have to get cross-strike faults, oblique faults, and other fractured zones in order to go across and through units such as a Conococheague, Elbrook, and Martinsburg Formations. The gradient is great, but can be slowed if the ground water encounters those units unless they are faulted. In the Greenbrier Valley which is at the edge of the Applachian Plateau and adjacent to the Valley the Ridge the faulting and folding is still influential to the gradient, but the dips are typically not as steep.
Geometry of Cave Passages:
Great Valley Caves: Great Valley Caves are characterized by development below the water table, taking on a phreatic shape, and following closely the strike of the bedrock. Also network caves are common due to the fracturing in the area, as well as thin-permeable sandstones that can occur in some of the units (Palmer, 2009). These caves are called networks because in cross section they take on a city street appearance (Palmer, 1991).
Greenbrier Caves: Greenbrier Valley caves that form on the edge of plateau also can feature network cave morphologies due to joints and fractures (Palmer, 2009). Both maze and network caves can occur under thin sandstone bands such as the base of the Mauch Chunk Group. There are two notable differences between caves in the Great Valley and caves in the Greenbrier Valley. 1) The caves in the Greenbrier Valley are more bedding plane controlled so they take on a sinuous branchwork pattern. 2) Although strike and dip play in a big role in the cave direction where there are variations in the strike and dip their control on cave passage development is less clear versus the Great Valley Caves (Palmer, 2009).
Great Valley Caves: Great Valley Caves are characterized by development below the water table, taking on a phreatic shape, and following closely the strike of the bedrock. Also network caves are common due to the fracturing in the area, as well as thin-permeable sandstones that can occur in some of the units (Palmer, 2009). These caves are called networks because in cross section they take on a city street appearance (Palmer, 1991).
Greenbrier Caves: Greenbrier Valley caves that form on the edge of plateau also can feature network cave morphologies due to joints and fractures (Palmer, 2009). Both maze and network caves can occur under thin sandstone bands such as the base of the Mauch Chunk Group. There are two notable differences between caves in the Great Valley and caves in the Greenbrier Valley. 1) The caves in the Greenbrier Valley are more bedding plane controlled so they take on a sinuous branchwork pattern. 2) Although strike and dip play in a big role in the cave direction where there are variations in the strike and dip their control on cave passage development is less clear versus the Great Valley Caves (Palmer, 2009).