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I I I I I I I I I I I I I I I I I I I 4.0 SUBSURFACE CONDmONS 4.1 Geology The regional geological units in the general project area consist of a thin layer of alluvial, eolian, and colluvial deposits of Quaternary age overlaying sandstone and shale units of Hunter Canyon Formation of Cretaceous age. The geology of the project area is shown on Figure 3.3. The river valley alluvial and eolian deposits consisting of silt, sand, gravel and boulders and can be found along the flood plains of Plateau Creek and the Colorado River. Colluvial deposits consisting of earth flow, mud flow, and talus deposits form the valley bottom and slopes. The Hunter Canyon Formation is of Cretaceous age and consists primarily of buff and gray, medium to coarse-grained, massive, cliff forming sandstone, and gray to greenish gray shale, with occasional thin coal layers. 4.2 Soil The overburden soil in the general project area consists of alluvial, eolian and colluvial deposits. The alluvial and eolian deposits are generally found in river valleys and along the Plateau Creek and the Colorado River flood plains. The alluvial and eolian deposits are estimated to be 5 to 35 feet in thickness. These deposits are mainly comprised of sand, gravel, with occasional layers of boulders and cobbles. Generally, the alluvial and eolian deposits can be classified as gravelly, silty sands, and sands and gravels in accordance with Unified Soil Classification System (USCS). The colluvial deposits are generally present covering the benches and gentler slopes of the canyon in this area. The average thickness of this colluvial soil is approximately two to five feet. These deposits are generally comprised of silt and clay derived from thin shale interbeds and sand and subangular sandstone fragments derived from massive sandstone units in the area. At several locations near the existing tunnel earth-flow debris has emplaced approximately 10 to IS feet of soil consisting of large sandstone blocks up to three feet and averaging six inches in maximum dimension mixed with a chantic, assemblage of sand, silt and clay. Generally, the colluvial soils can be classified as a silty or gravelly sand in accordance with USCS. 4.3 Bedrock The bedrock underlaying the project area consists of sandstone and shale units of Hunter Canyon Formation. The thickness of the individual sandstone units reach up to tens of feet and the thickness of the interbedded shales varies from one to two inches up to approximately three feet. 12