2006-12-15_REVISION - M1978208 - Laserfiche WebLink

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SITE GEOLOGY Most of the undisturbed area of the property is underlain by 0 to 3 feet of colluvial material derived from the underlying bedrock and occurinq pre- dominantly as a thin veneer, thickening locally in topographic lows. At the extreme western edge of the site alluvial material, deposited by Elk Creek, is in contact with the hillside colluvium. These surficiaT deposits are of Quaternary age, the alluvium being Pleistocene and the colluvium being Pleistocene to Holocene (recent). The bedrock underlying the surficial material is part of the Precambrian Idaho Springs Formation which makes up a large part of the uplifted crystalline rocks of the Front Range of the Centro) Rocky Mountains. At this particular locality,'the~Idaho Springs Formation is made up of migmatitic biatite gneiss and biotite schist interlayered with amphibolite and granitic gneiss. The gran- itic material occurs in numerous layers and lenses throughout the rock. The gneisses and schists are interlayered with the amphibolites and granitic materials to the extent that well developed compositional layering, usually parallel to sub-parallel with a less well developed foliation, is present striking variably from north-south to slightly northwest or slightly northeast. The dip of the compositional layering and the foliation is variable and in places undulating or erratic, but is predominantly 15a to 50o east (back into the hillside). The thickness of the compositional laminae varies from 1/2 inch to i2 inches or more where the granitic gneiss occurs in thick lenses. Near the surface biotitic gneisses and schists are weathered to the point where they can be scooped out by hand. At depth the degree of weathering decreases but the units are rippable. The amphibolite and granitic gneiss are less susceptible to weathering than the biotitic rocks and are more difficult to excavate. Blasting may be required in localities where amphibolitic -2-