Jamestown Dam - Geology

Geology

The Bureau of Reclamation details the geology of the reservoir area:

The reservoir area is a gently rolling plain with a surface blanket of glacial till over-lying thin-bedded, highly jointed Pierre Shale. In this plain the James River has cut a valley 1,000 to 4,000 feet wide and 50 to 100 feet, which forms the reservoir. The overburden of the reservoir area consists of three types:

1) Glacial deposits are chiefly till, a heterogeneous mass of buff to light-brown clay, silt, sand, gravel and boulders. Most of these materials are compact and impermeable.

2) Alluvium deposits occupying the valley floor range from highly plastic clays to sands and grovel with some silt. These deposits range in thickness from a few feet near the base of the valley to as much as 120 feet in the inner gorge near the center of the valley.

3) Deposits of slope-wash and slump materials are minor and occur as thin coverings along the valley slopes. These deposits consist chiefly of materials eroded from glacial drifts.

The bedrock is Pierre Shale which is normally hard, dark gray, thinly bedded and highly jointed claystone or siltstone.