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~J <br />July 28, 1980 <br />• <br />MINE: ROCKCASTLE-GRASSY CREEK - Sections 20, 21, 28, 29, 31, 32, <br />TSN, R87W, 6th P.M., Rou tt County, Colorado <br />SUBJECT: Area study for possible designation of alluvial valley floors <br />AREA <br />DEFINED: General - Basically the area within TSN, R87W <br />Specific - Mine area plus 2 miles downstream <br />GENERAL <br />AREA: Stream bottoms and drainage areas around the Rock castle-Grassy <br />Creek mine were examined for irrigation procedure and potential. <br />Sage Creek, although a larger creek than Grassy, has similar <br />valley floors as to size and shape. Upper parts of Fish Creek <br />are also similaz. <br />SPECIFIC , <br />AREA: The surficial deposits in the Grassy Creek valley floor have <br />been mapped from its headwaters to a .point 2 miles downstream <br />from the mine boundary. In the upper part of Grassy Creek <br />the valley is "V^ shaped and the valley floor is quite narrow, <br />often less than 50 feet wide. The deposits here in the upper <br />pazt consist of oolluvial material in the form of toe wedges, <br />landslide debris, and slope wash. In the actual bottom the <br />eazth material is probably 10 to 20 feet thick. <br />The valley floor widens immediately after passing through out- <br />crops of Cretaceous Twentymile Sandstone (just south of the <br />center of section 16). Earth material here is composed of a <br />mixture o~ alluvial fan deposits, colluvial slope wash and <br />possibly alluvial sand and gravel. The valley floor here <br />varies in width from 50 to 500 feet and is about 3000 feet <br />long. It is bounded on the east by the large fill of Routt <br />County road #27. Earth material in the valley bottom is <br />probably 30 plus feet thick. The actual stream channel is <br />15 to 25 feet wide and is up to 20 feet deep with neazly ver- <br />tical banks. Side drainages that are tributary to Grassy Creek <br />during runoff have cut arroyos to equivalent depths. This <br />stream entrenchment has left the remainder of the valley floor <br />high and dry and terrace-like in landform. The terraces are <br />not part of the active floodplain. <br />• The Grassy Creek valley floor for the next mile downstream <br />varies in width from 15 to 150 feet. Earth material in the <br />floor is a mixture of colluvial and alluvial deposits 10 to <br />20 feet thick. <br />V-13 <br />