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22 usually accumulate down-wind of dunes and other arid desert environments. Some engineering properties of eolian soils are listed below:  Prone to hydro-collapse  Can be liquefiable  Poor resistance to erosion  Sand deposits can be a source for fine aggregate  Low expansive  Easy to excavate Glacial drift is a general term that describes material deposited by glaciers. There are many terms used to describe the particular deposits from glaciers, but the two main modes of deposition are either directly from the glacial ice (these deposits are called till), or from streams of melt water (these deposits are called outwash). Generally, glacial till is deposited when the glacier retreats and dumps sediment as ice melts. This sediment, sometime called moraine, is defined by where it was deposited. For example, terminal moraine is deposited at the furthest downstream front of the glacier as it begins to retreat; lateral moraine describes the material deposited on the sides of the glacier as it retreats. Glacial till is extremely heterogeneous, non-stratified, poorly sorted, and contains particles of all sizes that are angular to sub- rounded. Glacial outwash deposits are similar to alluvial deposits and consist of stratified gravel and sand layers. Due to transport by flowing water, these deposits have well- sorted and rounded to sub-rounded particles. Some engineering properties of glacial drift are listed below. Glacial Till  Permeable to highly permeable  Extremely heterogeneous mix  Good source of aggregate  Difficult to excavate  Good bearing capacity Glacial outwash  Highly permeable  Can be liquefiable  Good source of fine aggregate  Good bearing capacity  Low shrink and swell potential Dam Issues Related to Geology Rock Foundations In most cases, foundations composed of igneous or metamorphic rocks are well-suited for construction of embankment dams. Igneous and metamorphic rocks are generally much stronger than the soil materials used to construct embankment dams. The stability of foundations and abutments for igneous or metamorphic foundations are mostly a function of the degree of jointing and weathering. Geologic investigations for rock dam sites focus the potential effects of those features on the structures. Perhaps the most significant problem associated with embankment dams with foundations composed of igneous or metamorphic rocks is related to seepage. Seepage could simply be a water loss problem and not necessarily a dam safety issue when the reservoir basin and abutments cannot hold water. Additionally, when an earthen embankment is constructed on top of a fractured bedrock foundation, care must be taken to prevent internal erosion and piping. When fractures and joints in the foundation are wide enough and continuous, seepage passing through the embankment becomes concentrated into these rock defects. This concentrated seepage induces high seepage velocities/forces that can lead to mobilization (internal erosion) of the overlying embankment material into the foundation. This condition was not well recognized by early dam engineers and has become a prominent failure mode during risk assessments of older dams. Problematic types of sedimentary rock for dam foundations include shales, mudstones, and other fine- grained rocks. One of the reasons fine-grained sedimentary rocks are problematic is the relatively low strength material that composes them. Geologically young, clay-rich fine-grained sedimentary rock found in much of the west has the tendency to slake. Slaking occurs in fine-grained sedimentary rocks when they are exposed to air. Slaking essentially causes a rock to deteriorate and breakdown. Some types of shale and mudstones begin to slake immediately after they are exposed to air. With shale, the process of slaking usually results in cracking along thin shale layers