The Ridgeley sandstone is a sandstone or quartzite of Devonian age found in the Appalachian Mountains of Pennsylvania, Maryland, West Virginia, and Virginia, United States. The Ridgeley is fine-grained, siliceous, calcareous in its lower strata, sometimes fossiliferous, and sometimes locally pebbly or conglomeritic. Varying in thickness from 12 to 500 feet (4–150 m), this rock slowly erodes into white quartz sand that often washes or blows away, but sometimes accumulates at large outcrops. When freshly broken, the rock is white, but outcrop surfaces are often stained yellowish by iron oxides.
The Ridgeley Sandstone was described and named in 1913 from an outcrop in Ridgeley, West Virginia, across the North Branch of the Potomac River from Cumberland, Maryland. It is currently recognized as the Ridgeley Member of the Old Port Formation; it has often been called the Oriskany sandstone in the region, but that name is based on an outcrop at Oriskany Falls, New York, and its application to these Central Appalachian rocks is debatable.
Outcrops of the erosion-resistant, ridge-forming Ridgeley ("Oriskany") sandstone are conspicuous landscape features in the Ridge and Valley physiographic province from south-central Pennsylvania through western Maryland and eastern West Virginia to Craig County in western Virginia. Ridgely outcrops form part of the lateral structures of North Fork Mountain, New Creek Mountain, and Wills Mountain of the Wills Mountain Anticline, although it is the even more erosion-resistant Silurian-aged Tuscarora Quartzite, not the Ridgely sandstone, that caps most parts of the prominent central ridge of this long geological structure. Other conspicuous Ridgeley outcrops include Caudy's Castle (Castle Rock) and Ice Mountain, both in Hampshire County, West Virginia.
At Berkeley Springs, West Virginia, warm water flows from natural mineral springs in the Ridgeley sandstone of Warm Springs Ridge; spa facilities (now in Berkeley Springs State Park) have long used these waters.
- "Geologic Unit: Ridgeley". National Geologic Maps Database (GEOLEX database). U.S. Geological Survey. Retrieved 30 September 2011.
- "Geologic Unit: Old Port". National Geologic Maps Database (GEOLEX database). U.S. Geological Survey. Retrieved 30 September 2011.
- "Oriskany Group including Ridgely Sandstone and Shriver Chert". Mineral Resources On-line Spatial Data [Maryland]. U.S. Geological Survey. Retrieved 30 September 2011.
- Means, John (2010). Roadside Geology of Maryland, Delaware, and Washington D.C. Missoula, Montana: Mountain Press. pp. x + 346.
- "Oriskany Sandstone and Helderberg Group, Undifferentiated". Mineral Resources On-line Spatial Data [West Virginia]. U.S. Geological Survey. Retrieved 30 September 2011.
- Cardwell, Dudley H., Robert B. Irwin, and Herbert P. Woodward, with cartography by Charles W. Lotz (1968, slightly revised 1986). Geologic Map of West Virginia. West Virginia Geological and Economic Survey. p. 2. Check date values in:
- "Ridgeley Sandstone, Helderberg and Cayugan Groups". Mineral Resources On-line Spatial Data [Virginia]. U.S. Geological Survey. Retrieved 30 September 2011.
- Tilton, John L., William F. Prouty, R.C. Tucker, and Paul H. Price (1927). Hampshire and Hardy Counties. Morgantown, West Virginia: West Virginia Geological Survey. pp. xiii + 624.
- Originally spelled "Ridgely" but changed to "Ridgeley" to conform to usage by the U.S. Postal Service.