1978-06-22_PERMIT FILE - M1978208 (10)

Home Browse Search

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 occuring 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 surficial deposits are of Quaternary age, the alluvium being Pleistocene and the colluvium being tPleistocene 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 Central Rocky Mountains. At this particular locality, the Idaho Springs Formation is made up of migmatitic biotite 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 150 to 500 east (back into the hillside). The thickness of the compositional laminae varies from 1/2 inch ' to 12 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 bictitic rocks and are more Idifficult to excavate. Blasting may be required in localities where amphibolitic -2-