Parnell Knob is a mountain in the Ridge and Valley Appalachians of south-central Pennsylvania, overlooking the village of St. Thomas where Front Mountain and Broad Mountain meet. Its striking appearance comes from rising sharply above the relatively level floor of the Great Appalachian Valley, making it a prominent landmark in the surrounding landscape. The mountain’s form was shaped from hard, dense Tuscarora quartzite dating to the Silurian age, and the durability of that resistant rock helps account for both its abrupt profile and its lasting visibility.