Cave Development in Strata of Ordovician-and Silurian-Devonian-Age in Highland County, Virginia

Abstract

Picturesque Highland County, Virginia, also known as Virginia's Little Switzerland, is characterized by high mountains, tranquil rivers, and hundreds of caves. This study determines how geologic structures and processes control speleogenesis, or cave development, in the county. Solutional caves in Highland County are found in Ordovician limestones and dolostones and in Silurian- to Devonian-age limestones. Despite the lithologic and structural differences between the strata, caves in both sections tend to be similarly joint-controlled in directions of both regional strike (N40°E), dip (northwest or southeast), or in fractures intersecting at 60 and/or 120 degrees. Brittle failure, including fractures and faults induced by folding, appears to be the most prominent controlling factor of speleogenesis in Highland County. Despite the findings of other studies indicating that branchwork cave patterns dominate most karst aquifers by frequency and total cave length, fissure-type caves are the most frequent pattern in Highland County and maze network caves are the most predominant pattern by total cave length. Fissure-type caves tend to be very short with most occurring up to 20' (6 m) long. Slot fissures are more canyon-like and tend to be longer with most being either 21-40' (6-12 m) long or 101'-200' (31-61m) long. Branchwork caves tend to be longer with most ranging from 201'-300' (61-91 m) long, while maze networks tend to be the longest, with most ranging from 801'-2000' (244-610 m) long. Though cave patterns in Highland County do not entirely reflect predominant cave patterns found worldwide, the overall trend of cave lengths is similar to that found throughout Virginia. In addition to the aforementioned cave patterns found in Highland County, pits and rooms also constitute a large proportion of the cave types in the county, evidence of the vadose recharge that most affects the subsurface dissolution. Approximately 92% of Highland's caves show vadose cave development, while only 4% show active phreatic development, and 5% show characteristics of both active vadose and phreatic development. These observations are consistent with the fact that fissures, slot fissures, pits, and rooms—the types of caves most frequently found in Highland County—all form in areas of vadose recharge.