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Sandstone which is composed of K3 and Kq. K5 and K6 are located above the Twentymile Sandstone. The K6 horizon marks the top of the Willioms Fork Formation. Although the K notation is very useful for referenc ing siratigraphy, it must be noted that all six key beds may not persist throughout the study area. Lewis Shale The sandy shale and Fish Creek coal immediately above K6 are considered The base of the Lewis Shale. Other workers have included the Fish Creek coal with the Williams Fork, which has led to some serious problems in correlation. The Lewis Shale (Kls on Exhibit 2.1-I) is o medium to dark grey marine shale also of Upper Cretaceous age. It has a thickness in excess of 1,000 feet in the vicinity of the study area. Browns Pork Formation The Browns Park Formation (Tbp on Exhibit 2.1-I) unconformably overlies the Upper Cretaceous sediments discussed above. It is chiefly composed of terrestially • deposited volcanically derived sediments 20 to 24 million years in age (Miocene). In the vicinity of the study area, the Browns Park is a light brown to white coarse grained, conglomeratic sandstone that is poorly indurated and may attain a thickness of 300 feet. The Browns Park is exposed in the northeastern corner of Exhibit 2.1- I where it is in contact with the Williams Fork Formation, the Iles Formation, and the Mancos Shale. Tertiary Age Volcanics Volcanic rocks of basaltic composition are exposed in the study area (Tv on Exhibit 2.1-I). These rocks are thought to represent volcanic flows that are 9 to 14 million years in age (late Miocene). These volcanic rocks may correlate with basalt flows in the Fldt Tops, which are located southwest of the study area. Both the Browns Park and the Tertiary volcanics were once far more extensive and have been eroded away leaving behind a discontinuous mantle of basaltic cobbles and boulders that are found throughout The region. 2.1-3