| Materials within and surrounding the area of Willow Creek Dam and Reservoir include Cretaceous and Paleocene sedimentary rocks, Miocene basalt flows with interlayered mudstone, siltstone and sandstone of the same age, Pleistocene terrace deposits and alluvium and terrace deposits of Holocene age. The dam and appurtenant structures are situated in an area of Miocene basalt flows and interlayered tuffaceous mudstone, siltstone and sandstone. Most of the older sedimentary rocks, as well as younger surficial materials occur within the upper reaches of the reservoir. At the time investigations were conducted, the sedimentary rocks interlayered with Miocene basalt flows were refereed to as the North Platte formation. Since that time there has been a change in nomenclature. The subject sedimentary rocks are now called the Troublesome formation. |
