Scenic Saturday: Pinnacle in the Piedmont

Big Pinnacle at Pilot Mountain State Park, North Carolina. May 2009.

The peak of Big Pinnacle at Pilot Mountain State Park rises more than 450 m above the surrounding North Carolina Piedmont. Big Pinnacle is just the most eye-catching of series of peaks, called the Sauratown Mountains, that are a tectonic window through thrust sheets from the Alleghaninan orogeny. You can see some of the other peaks in the background of the photo above – and how the whole thing is a big anticline.

Light colored quartzite forms vertical cliffs at Pilot Mountain State Park, North Carolina. May 2009.

The vertical sides of the Big Pinnacle are a result of the erosion-resistance of very pure quartzite, that originated as beach sand about 540 million years ago. A bit lower in the stratigraphic column, there’s also some mica schist, and where the two are exposed next to each other, they make a cool contrast. The small folds in the schist mimic the shape and orientation of the big anticline.

Light colored quartzite and darker, folded schist at Pilot Mountain State Park, North Carolina. May 2009. Apologies for lack of scale.

Metamorphic rocks, like the ones at Pilot Mountain, have extremely low permeability, because there’s no space between crystals for water to squeeze through. The only way you get any water out of the crystalline rocks of the Piedmont is by drilling into a fracture. On the lower slopes of Pilot Mountain, one of these water-filled fractures reaches the surface and forms a small spring, called “Ledge Spring.” On a hot NC spring day, the spring made a perfect resting spot for some weary hikers, and a thirsty dog, before climbing back up the mountain toward the car that would take us back to much flatter parts of the Piedmont.

Ledge Spring, near the base of Pilot Mountain, North Carolina. May 2009.