East River Mountain is a mountain ridge that forms part of the border between Mercer County and Virginia. It runs for about 35 miles until the New River cuts through it at a spot called The Narrows.

The mountain's crest is made of a really hard, durable rock called Tuscarora Sandstone. It’s so tough that water and weather haven't been able to carve any deep gaps into the ridge. Geologists call the mountain's structure anticlinal—meaning the rock layers are bent upward into a giant arch shape.

From the top (the high point is 4,069 feet), you can see a big difference:

• To the southeast (Virginia), you see the Ridge and Valley region with long, parallel mountains and valleys.

• To the northwest (West Virginia), you see the Allegheny Plateau with wider, flatter tops and winding streams.

This big, unbroken mountain was a huge problem for travelers for centuries. That barrier was finally broken in 1974 with the construction of the East River Mountain Tunnel, which allows I-77 to pass through into Virginia.