The wilderness is located a few miles north of the continental divide. The rock strata forming the wilderness are tilted upward to the north. To the south are Mississippian layers, formed between 320 and 360 million years ago. On the upper reaches of the mountain is tough Devonian sandstone, formed between 360 and 408 million years ago. It caps Brush Mountain and juts out to the north at the top of the wilderness slope. Below it lies brittle layers of older and more loose Devonian shale which is subject to erosion. The lower reaches along Craig Creek valley contain Silurian sandstone, formed between 408 and 438 million years ago and extending northwest to form Sinking Creek Mountain.