Purcell Knob is a 1,207-foot mountain spur of the Blue Ridge in Loudoun County, Virginia, near Keyes Gap and the village of Neersville. Though modest in height, it is significant for an exposed section of the sericitic phyllite base of the Paleozoic Loudoun Formation. Visible in an antiformal syncline, this geological feature connects the summit area to the older rock structure of the Blue Ridge and gives the mountain importance beyond its elevation alone.