1987-12-01_PERMIT FILE - C1981017 (172)

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• Mancos Shale : Within the Mesaverde formation , the Mancos Sha-ie overlieing the Cozzette Sandstone member is somewhat more fossiliferous than the underlieir_g Mancos Shale , but otherwise identical . It is fine grained , soft , and erodes easily in the form of mud and silt . Rollins Sandstone : More coarse grained and resistant than the shales , it forms a nearly vertical cliff at outcrop , around the basin. The Rollins erodes in large blocks and boulders . Bowie Shale : Generally more competent than the Mancos Shale because it contains more sandstone , the Bowie is commonly exposed in cliff faces around the basin. The unit is well exposed , so weathering and erosion is intense . Middle Sandstone : Occurs , like the Rollins , as a nearly vertical cliff face at outcrop , around the basin . It is nearly identical to the Rollins member , and erodes in large blocks and boulders . Paonia Shale : More coarse grained and resistant to erosion than the other shales , the Paonia does , how- ever , exhibit the same high erosional tendency . Upper Sandstone : Occurs , like the Rollins and Middle Sandstone members , as a nearly vertical cliff face at outcrop , around the basin . It is nearly identical to the other Rollins and Middle members , also eroding in larger blocks and boulders . Upper Williams Fork: Forming the upper rim of the basin , the undifferentiated , upper Williams Fork member erodes in very large blocks and boulders . Mechanical weathering predominates due to the steepness of the exposures . Tertiary (Eocene) igneous rocks in the basin erode more readily than would ordinarily be expected . Fragments of dike rock are fairly common in float and stream bed material in the parts of the basin where intrusions occur . Valley glaciation probably had some effect on . the topography of the basin , and on the west side of Hunts- i4