Hagerman Fossil Beds National Monument - Geology

As the Snake River flows through the Hagerman Valley it cuts between nearly horizontal sedimentary geological deposits. The geological formations on the Monument consist largely of the sedimentary deposits of the Glenns Ferry Formation and several minor basaltic lava flows. The Hagerman Valley lies near the eastern extremity of the Glenns Ferry Formation, a complex mixture of flood plain, fluvial (stream), and lacustrine (lake) deposits associated with ancient Lake Idaho. The Glenns Ferry Formation is interbedded with local, discontinuous lava flows, which extends over several thousand square miles of the western part of the Snake River Plain. The deposits of the Glenns Ferry Formation have proven to be highly fossiliferous.

Plateaus are topped with a permeable layer of gravel and sand known as the Tuana Gravel, and a less permeable layer of calcium carbonate known as caliche. This continuous, several feet thick, layer of caliche caps the Pleistocene-aged Tuana Gravel. It functions as a caprock on the top of the bluffs and where unbroken, acts as a significant barrier to ground water infiltration. Soils here are classified as shallow to very shallow making their available waterholding capacity very low.

To the east and north of Hagerman Valley, extensive basalt flows of various ages cover a larger area of the Snake River Plain than do the sediments. These flows are younger than the Glenns Ferry Formation and cover the Glenns Ferry Formation in outcrops along Billingsly Creek. Five separate basalt flows form the cliffs on the east side of the valley. From the geology of the valley, a generalization can be made that valley walls and land to the east of the Snake are primarily igneous in nature (the Snake River Group), while strata to the west is primarily sedimentary (the Idaho Group).

The valley floor consists of varying geological features. The Miocene Banbury Basalt forms the rapids at Upper and Lower Salmon Falls and probably Bell Rapids, now submerged by the reservoir. The Sand Springs Basalt, which erupted from Flat Top Butte just north of Twin Falls, forms the main floor of the valley. Parts of the valley floor are covered by surface deposits distributed by the Snake River and are dominated by the Melon Gravel deposited during the Lake Bonneville flood 15,000 years ago.