REMARKS: Well-preserved columnar joints in the Catoctin metarhyolite (Precambrian age) are exposed along the west wall of the dam and spillway. The reservoir serves as a water supply for the hospital at South Mountain. Unique columnar structures exposed here are about 20 feet high and as much as 2 feet wide, and have hexagonal and pentagonal sides. Tree cover behind the spillway wall is sufficiently thick during the summer months to completely hide this feature.
REFERENCE: Pennsylvania Geology (1969), Columnar jointing in South Mountain, v. 1, no. 2, p. 7.
REMARKS: A mass of diabase boulders facing Little Round Top (336) and Round Top (337). Thousands of sightseers have climbed these rocks on the Gettysburg battlefield to look over the fields where Pickett's men charged on July 3, 1863. Very few of these Civil War buffs know that these geologic features that Generals Lee and Meade fought among are the outcrop of a diabase sill, called the Gettysburg sill. The sill intruded into the Triassic red sandstones and shales that floor the broad Gettysburg Valley 180 million years ago. Few have any idea of the extent to which the battle was influenced by the geology of the region.
REFERENCE: Brown, Andrew (1962), Geology and the Gettysburg campaign, Pennsylvania Geological Survey, 4th ser., Educational Series 5, 15p.
REMARKS: A water gap formed by Conewago Creek; has a macimum relief of 680 feet. Large and numerous outcrops of gray-green metarhyolite (Pre-cambrian age) dot the surface; note the flow structures and various sized porphyritic textures.
REFERENCE: Shirk, W. R. (1978), The geology of south-central Pennsylvania, Guidebook, National Association of Geology Teachers, Eastern Section, Shippensburg State College, Shippensburg, Pennsylvania, 73p.