The Porter Peak Limestone was named for exposures on Porter Peak, in Sec. 28, T. 44 N., R. 51 E., in the northern portion of the Bull Run Quadrangle in the northern Bull Run Mountains (Decker, 1962).

The Porter Peak is considered Upper Cambrian in age on the basis of algal, coral, and brachiopod debris (Decker, 1962). In the Bull Run Mountains it conformably overlies the Edgemont Formation.

The Porter Peak Limestone can be divided into three portions. The upper unit is composed of massive blue-gray limestone which is locally altered along intrusives to a light-cream marble (Decker, 1962). The lower third of the formation is light gray, sandy limestone interbedded with mottled black dolomitic limestone. The basal portion of the formation is composed of two, massive, gray, limestone-pebble conglomerates with rounded limestone pebbles. The upper conglomerate contains sub-rounded limestone, siltstone and shaly limestone clasts up to 2 feet across while the lower limestone contains rounded gray limestone pebbles up to 2 inches in diameter (Decker, 1962). The conglomerates are separated with about 150 feet of gray siltstone and black shaly limestone.

The Porter Peak is estimated to be 3,200 feet thick at the type section in the Bull Run Mountains (Decker, 1962).

The Porter Peak Limestone is exposed in the Bull Run Mountains and may be partially equivalent to Paleozoic schist and limestone mapped by Bushnell (1967) in the Rowland Quadrangle.

Decker (1962) considered the Porter Peak to represent a near-shore facies of an algal bioherm based upon the presence of limestones with intraformational conglomerates, and shallow warm water forms such as corals and algae. Essentially nothing else has been done to detail the depositional environment for the Porter Peak.