Toby is camping in a broad, u-shaped valley. the area was most likely created by

The answer is glacial erosion.

U-shaped valleys, as named, have a characteristic U shape. Their evolution usually start as a river valley. Thus, they have V shape before erosion occurs. When glacier start its travel down a slope, it erodes and deepens that V-shaped valley. Over the time, valley get the U shape and becomes U-shaped valley, also called glacial troughs.