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The outcropping strata in the Coal Basin <br /> • differ in erosional properties but are , for the most part , <br /> highly erosive, particularly in the case of the Mancos <br /> Shale, which floors most of the basin and is the most common <br /> unit . <br /> Because the shale is easily eroded , it tends <br /> to be highly erratic . Moreover , several areas within the <br /> unit have been contorted by plastic deformation , thus increasing <br /> the erratic tendencies . The shale is fine grained and soft . <br /> On exposure it breaks into small , shaley fragments , contributing <br /> great quantities of light brown to gray colored mud and silt <br /> during periods of high runoff . <br /> The shales of the overlieing Mesaverde forma- <br /> tion, including the 1 , 000 ' "tongue" of Mancos shale , are <br /> also highly erodable , and erratic . The sandstone members <br /> exhibit greater competence , and are distinguishable as <br /> nearly vertical cliffs . The upper , undifferentiated "Williams <br /> Fork forms the resistant upper rim of the basin, Huntsman <br /> Ridge, which, however , exhibits the effects of mechanical <br /> weathering . <br /> Specifically , the members of the Mesaverde <br /> Formation are characterized by the following erosional <br /> tendencies : <br /> Cozzette Sandstone : fine grained and scarcely dis- <br /> tinguishable from the Mancos Shale . Tt is , however , <br /> slightly more resistant to erosion , and forms ledges <br /> and waterfalls at outcrop . <br /> • <br /> 13 <br />