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~.. iii iiiiiiiiniiiiii 999 July 28, 1980 MINE: ROCKCASTLE-GRASSY CREEK - Sections 20, 21, 28, 29, 31, 32, TSN, R87W, 6th P.M., Routt County, Colorado SUBJECT: Area study for possible designation of alluvial valley floors AREA DEFINED: General - Basically the area within TSN, R87W Specific - Mine area plus 2 miles downstream GENERAL AREA: Stream bottoms and drainage areas around the Rock castle-Grassy Creek mine were examined £or irrigation procedure and potential. Sage Creek, although a larger creek than Grassy, has similar valley floors as to size and shape. Upper parts of Fish Creek are also similar. SPECIFIC AREA: The surficial deposits in the Grassy Creek valley floor have been mapped from its headwaters to a point 2 miles downstream from the mine boundary. In the upper part_of Grassy Creek the valley is "V" shaped and the valle~floor_is_quite_nairow, ~- often less_than 50_feet_wide.. The_.deposits_here_in_the upper part consist of colluvial_material in _the_form of toe wedges, landslide_debris,_and_slope_wash. In the actual bottom the earth material is probably 10 to 20 feet thick. - The valley floor widens immediately after passing through out- crops of Cretaceous Twentymile Sandstone (just south of the - center of section 16). Earth material here is composed of a mixture of alluvial fan.deposits, colluvial slope wash and ,.. possibly alluvial sand and gravel. The valley floor here varies in width from 50 to 500 feet and is about 3000 feet long. It is bounded on the east by the large fill of Routt County road #27. Earth material in the valley bottom is probably 30 plus feet thick. The actual stream channel is 15 to 25 feet wide and is up to 20 feet deep with nearly ver- tical banks. Side drainages that are tributary to Grassy Creek during runoff have cut arroyos to equivalent depths. This stream entrenchment has left the remainder of the valley floor high and dry and terrace-like in landform. The terraces are not part of the active floodplain. The Grassy Creek valley floor for the next mile downstream varies in width from 15 to 150 feet. Earth material in the floor is a mixture of colluvial and alluvial deposits 10 to 20 feet thick.