This week's Friday Field Photo features a Silurian (444-416 million years ago) sandstone called the Tuscarora Formation on display along the Appalachian Trail near Roanoke, Virginia. In this location, the beds are dipping very steeply, almost vertical, with stratigraphic 'up' to the right. The popular Dragon's Tooth and McAfee Knob hikes, both along the Appalachian Trail, follow ridges held up by this robust sedimentary unit. These are fun hikes because the trails will follow the same set of beds for long distances providing spectacular views of the adjacent valleys.
These trails are in an area called the Valley and Ridge province, which you can get a sense of in the topographic map below. More resistant sedimentary rocks like the Tuscarora Formation or other limestone formations create ridges whereas rocks like mudstone and siltstone are more easily worn down and create valleys. These sedimentary rocks were crumpled up into these folds during a mountain-building event some 100 million years after they were deposited. That 300 million year-old mountain range rivaled the modern Himalayas but has been slowly eroding ever since to form what we now know as the Appalachian Mountains.
It's great to be able to take a pleasant walk in the woods and ponder all that geologic history.
Images: (1) Sandstone beds along the trail / my Flickr page; (2) Topographic map of valley and ridge province near Roanoke, VA (created with free GeoMapApp tool)
