PROJ00357 - Laserfiche WebLink

Home Browse Search

I I. I I I I I I I I. I I I I I I I. I I . . CHAPTER II I LAND LAND DESCRIPTION The Grand Mesa Project service area is depicted in Figure 5-1. Broad mesas and narrow stream valleys are characteristic land forms within the area. Farm lands are well distributed over the area, though not all are irrigated. Most of the irrigated lands are situated on the gently sloping mesa tops. They have a general slope of 4 to 5 percent to the southwest. Numerous creeks and gullies dissect the mesas and serve both as water supply and for drainage. They flow southwesterly in a roughly parallel pattern through the Project area. Elevations vary from 5,400 feet in the lower stream valleys to 7,400 feet in the upper fields. The mesas were formed by the deposition of alluvial gravels and glacial till over the Mancos Shale which underlies the region. The alluvium originated from igneous parent materials - deposited as glacial drift and till _ during recent glacial periods. It appears that a lobe of an ice cap, centered on Grand Mesa, extended part-way down the Surface Creek drainage. Materials deposited by the ice have since been washed down by stream action to form an erosion-resistant layer which protects the softer shales below from erosion. In drainages other than Surface Creek, and in the lower reaches of Surface Creek, the alluvium thickness was less. It has since eroded completely allowing entrenchment of the streams into the Mancos Shale. Soils on the mesa tops have a medium texture with numerous cobbles. They are fairly well drained with low saline-sodic concentrations. Native vegetation consists of sagebrush and pinyon-juniper forest in non-farmed areas. The valley lands lie 200 to 500 feet below the mesa tops. The soils are derived from Mancos Shale and tend to be fine-textured and heavy. Some of the lower areas contain fairly high concentrations of soluble salts which are leached into return irrigation flows. Greasewood and rabbit brush are the -14-