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<br />I <br />I <br />I <br />I <br /> <br />I <br /> <br />I <br />I <br />I <br /> <br />I <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />I <br /> <br />- <br /> <br />eroded by East Divide Creek, the weak Wasatch claystone and siltstone members, <br /> <br /> <br />and probably a period of high precipitation in the form of rain and snow. <br /> <br />The surficial materials and topography suggest the following sequence of <br /> <br /> <br />geologic events in late Pleistocene and Holocene (recent) times: <br /> <br />1. <br /> <br />In probably late Pleistocene time, a very large landslide in the <br /> <br /> <br />Wasatch Formation moved northeastward down the slope of Mosquito <br /> <br /> <br />Mountain toward East Divide Creek. <br /> <br />2. <br /> <br />The landslide filled the canyon along about a 1-1/2 mile reach of East <br />Divide Creek and basically dammed the creek to form a fairly large <br />lake. <br /> <br />3. <br /> <br />Sediments filled the lake over many years to create the gentle slopes <br />seen as Reservoir Park at the present. <br /> <br />4. <br /> <br />The lake periodically spilled over the landslide blockage and <br /> <br /> <br />eventually eroded through the slide material until the lake was <br /> <br /> <br />largely drained. The re-established creek then cut the narrow canyon <br /> <br /> <br />along the east margin of the slide seen today. <br /> <br />The dam and reservoir area were explored by seven test pits excavated with <br /> <br /> <br />a backhoe on September 29, 1988. One pit (TP 3) was located on the left dam <br /> <br /> <br />abutment and found 5 feet of lean clay containing basalt cobbles and boulders. <br /> <br /> <br />The material was moist and well compacted. A fresh crack is present at the <br /> <br /> <br />ground surface approximately 80 feet north of this test pit (see Geology and <br /> <br /> <br />Borrow Area Map, Figure 5.1, for location). This crack represents the south <br /> <br /> <br />margin of a recent, localized slump in the ancient landslide mass and can be <br /> <br /> <br />traced for 300 to 400 feet up the hillside. The axis of the proposed dam is <br /> <br /> <br />some 80 feet up the valley from the crack and at a lower elevation. The <br /> <br /> <br />abutment where the dam will be anchored contains many large boulders which help <br /> <br /> <br />to provide stability against large-scale movement. <br /> <br />-13- <br />