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the Twentymile Park Syncline. Changes in stream regime since late Tertiary time • have produced episodic aggradation and degradation resulting in a complex melange of alluvial, strath and composite stream terraces. Alluvial terraces are depositional landforms that are composed of water-transported materials ranging in size from clay to boulders. They have been created by the incision of the stream through previously deposited valley fills. In contradistinction, strath terraces are composed chiefly of bedrock and have been formed by incision of the stream into bedrock. The resulting land form is a rock terrace often mantled with a thin veneer of soil and/or alluvium. A composite Terrace is formed when stream incision penetrates alluvial valley fill and erodes into bedrock. The resulting terrace is partially bedrock and partially alluvium. There are nearly as many levels of terraces above Trout Creek as there are terraces. Some of these terraces result from The glacial activity in the watershed, others are attributable to either baselevel changes in The Colorado River System, climatic change or land use related causes. For purposes of mapping, the terraces have been grouped into four classes described in the following paragraphs. The mapping of the terraces is shown in Exhibits 2.5-21 and 2.5-22. X11 terraces are the lowest and youngest. They represent The modern • floodplain of Trout Creek and are underlain by alluvium. The water table is close to the surface of The terraces throughout the year and hydrophytic vegetation is prominent. These terraces are hydrologically connected To Trout Creek. There are several different levels of Terraces included in The OTI unit. These terraces are diminutive and represent relatively recent lateral shifts of Trout Creek which is slowly degrading at most locations. These small terraces generally increase in height above the stream, in a downstream direction. A prominent example and the highest of these terraces is located south of Highway 33 aT Trout Creek and is the rile of The Adams residence. The OTI terraces are alluvial terraces composed chiefly of sand, gravel and boulders, locally covered by finer overbank deposits. The ~t2 terraces are most prominent on the west side of Trout Creek. They are composite terraces sculpted from The sandstones and shale of the Williams Fork Formation and, in many places, ore capped with eight feet or more of alluvium and colluvium. These terraces also increase in height above Trout Creek in a downstream direction. They range in height above the creek from 10 to 65 feet and are continuous along the west side of the creek. These terraces are discontinuous on the east side of Trout Creek where They are, to a large extent, missing. r 1 LJ 2.5-62