Geology of the Trout Rock caves (Hamilton Cave, Trout Cave, New Trout Cave) in Pendleton County, West Virginia (USA), and implications regarding the origin of maze caves
Authors: Swezey Christopher S. , Brent Emily L. 
Published: March 2020

The Trout Rock caves (Hamilton Cave, Trout Cave, New Trout Cave) are located in a hill named Cave Knob that overlooks the South Branch of the Potomac River in Pendleton County, West Virginia, USA. The geologic structure of this hill is a northeast-trending anticline, and the caves are located at different elevations, primarily along the contact between the Devonian New Creek Limestone (Helderberg Group) and the overlying Devonian Corriganville Limestone (Helderberg Group).

The entrance to New Trout Cave (Stop 1) is located on the east flank of Cave Knob anticline at an elevation of 585 m (1919 ft) above sea level, or 39 m (128 ft) above the modern river. Much of the cave consists of passages that extend to the northeast along strike, and many of these passages have developed along joints that trend ~N40E or ~N40W. Sediments in New Trout Cave include mud and sand (some of which was mined for nitrate during the American Civil War), as well as large boulders in the front part of the cave. Gypsum crusts are present in a maze section of the cave ~213–305 m (799–1001 ft) from the cave entrance. Excavations in New Trout Cave have produced vertebrate fossils of Rancholabrean age, ca. 300–10 thousand years ago (ka).

The entrance to Trout Cave (Stop 2) is located on the east flank of Cave Knob anticline ~100 m (328 ft) northwest of the New Trout Cave entrance at an elevation of 622 m (2040 ft) above sea level, or 76 m (249 ft) above the modern river. Much of the cave consists of passages that extend to the northeast along strike, although a small area of network maze passages is present in the western portion of Trout Cave that is closest to Hamilton Cave. Many of the passages of Trout Cave have developed along joints that trend N50E, N40E, or N40W. Sediments in Trout Cave include mud (also mined for nitrate during the American Civil War), as well as large boulders in the front part of the cave. Excavations in the upper levels of Trout Cave have produced vertebrate fossils of Rancholabrean age (ca. 300–10 ka), whereas excavations in the lower levels of the cave have produced vertebrate fossils of Irvingtonian age, ca. 1.81 million years ago (Ma)–300 ka.

The entrance to Hamilton Cave (Stop 3) is located along the axis of Cave Knob anticline ~165 m (541 ft) northwest of the Trout Cave entrance at an elevation of 640 m (2099 ft) above sea level, or 94 m (308 ft) above the modern river. The front (upper) part of Hamilton Cave has a classic network maze pattern that is an angular grid of relatively horizontal passages, most of which follow vertical or near-vertical joints that trend N50E or N40W. This part of the cave lies along the axis of Cave Knob anticline. In contrast, the passages in the back (lower) part of Hamilton Cave lie along the west flank of Cave Knob anticline at ~58–85 m (190–279 ft) above the modern river. These passages do not display a classic maze pattern, and instead they may be divided into the following two categories: (1) longer northeast-trending passages that are relatively horizontal and follow the strike of the beds; and (2) shorter northwest-trending passages that descend steeply to the west and follow the dip of the beds. Sediments in Hamilton Cave include mud (which was apparently not mined for nitrate during the American Civil War), as well as large boulders from the Slab Room to the Rosslyn Escalator. Gypsum crusts are present along passage walls of the New Creek Limestone from the Slab Room to the Airblower. Excavations in the front part of Hamilton Cave (maze section) have produced vertebrate fossils of Irvingtonian age (ca. 1.81 Ma–300 ka).

The network maze portions of Hamilton Cave are interpreted as having developed at or near the top of the water table, where water did not have a free surface in contact with air and where the following conditions were present: (1) location on or near the anticline axis (the location of the greatest amount of flexure); (2) abundant vertical or near vertical joints, which are favored by location in the area of greatest flexure and by a lithologic unit (limestone with chert lenses) that is more likely to experience brittle rather than ductile deformation; (3) widening of joints to enhance ease of water infiltration, favored by location in area of greatest amount of flexure; and (4) dissolution along nearly all major joints to produce cave passages of approximately the same size (which would most likely occur via water without a free surface in contact with air).

The cave passages that are located along anticline axes and along strike at the New Creek–Corriganville contact are interpreted as having formed initially during times of base-level stillstand at or near the top of the water table, where water did not have a free surface in contact with air and where the water flowed along the hydraulic gradient at gentle slopes. Under such conditions, dissolution occurred in all directions to produce cave passages with relatively linear wall morphologies. In the lower portions of some of the along-strike passages, the cave walls have a more sinuous (meandering) morphology, which is interpreted as having formed during subsequent initial base-level fall as cave development continued under vadose conditions where the water had a free surface in contact with air, and where water flow was governed primarily by gravitational processes. Steeply inclined cave passages that are located along dip at the New Creek–Corriganville contact are interpreted as having formed during subsequent true vadose conditions (after base-level fall). This chronology of base-level stasis (with cave development in the phreatic zone a short distance below the top of the water table) followed by base-level fall (with cave development in the vadose or epiphreatic zone) has repeated multiple times at Cave Knob during the past ~4–3 million years (m.y.), resulting in multiple cave passages at different elevations, with different passage morphologies, and at different passage locations with respect to strike and dip.