Figure 3.3-4. Structural Features and Bedrock Units of the Project Vicinity

Figure 3.3-4. Structural Features and Bedrock Units of the Project Vicinity

The Greater Platte River basins, including the North Platte, contain abundant surficial deposits that were sensitive to, or are reflective of, the climate under which they formed: deposits from multiple glaciations in the mountain headwaters of the North and South Platte rivers and from continental ice sheets in eastern Nebraska; fluvial terraces along the rivers and streams; areas of eolian sand and other dune fields; thick sequences of wind-blown silt (loess); and sediment deposited in numerous lakes and wetlands (USGS undated).

Thin surficial deposits including alluvium, colluvium, and in-situ decomposed parent rock (particularly in the igneous rocks) commonly occur in drainages and depressions, as well as on some side slopes within the Project vicinity (Bennett and Aalto 1982). A list of all prominent surficial units mapped throughout the Project vicinity are described in Table 3.3-1 and are shown on Figure 3.3-5 (WSGS 2020).

Table 3.3-1. Descriptions of Surficial Deposits in the Vicinity of the Project

Unit Surficial Deposits Abbreviations (1:100,000 scale)

a Alluvium - stream and river deposits. b Bench - a strip of relatively level earth or rock, raised and capped with gravel. c Colluvium - loose and incoherent deposits, usually at the foot of a cliff or on the surface of a slope and there chiefly by gravity. d Dissected.

e Eolian deposits - windblown deposits, includes sand, silt, and clay. f Alluvial fan deposits - a fan shaped deposit made by a stream or a debris flow where they have run out onto a level plain. l Landslide - earth and rock which became loosened from a hillside and slides, flows, or falls down the slope.

R Bedrock. r Residuum - a residual deposit remaining in place after the decomposition of rocks. s Slopewash - soil and rock material that has been moved down a slope by gravity assisted by running water. t Terrace deposits - relict alluvial deposits on relatively flat, horizontal, or gently inclined surfaces which are bounded by a steeper ascending slope on one side and by a steeper descending slope on the opposite side. u Grus - an accumulation of angular, coarse- grained fragments resulting from the granular disintegration of crystalline rocks. Source: WSGS 2020.

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