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erratic , deeply eroded terrain . Dips along the flanks of <br /> 0 <br /> the anticline vary from nearlv flat to 20 south of the <br /> 0 0 <br /> axis , and from 5 to 30 on the north side , while in the <br /> 0 <br /> lower Coal Creek drainages , dips range from 35 to nearly <br /> vertical . The center of the basin , which is floored by <br /> deformed and highly erosive Mancos Shale , is notably erratic . <br /> The vertical rise of ground within the basin <br /> is precipitous , ranging from 7 , 180 ' elevation where Coal <br /> Creek enters the Crystal , to 8 , 000 ' at the preparation plant <br /> area, to 10 , 000 ' at the portals , to 11 , 500 ' along the Hunts- <br /> man Ridge. Areas of moderate terrain are rare , occurring <br /> only in the south part of the basin and in a few landslide <br /> and slumpage areas . <br /> The Coal Basin is highly eroded . The erosion <br /> cycle probably dates from the end of the Eocene , some 36 , 000 , 000 <br /> years ago . It has been estimated that , assuming a fairly <br /> constant rate of erosion, some 15 , 225 tors of rock , including <br /> 29 tons of coal , are eroded out of the Coal Creek drainage <br /> system each year . Since the Eocene , the rock estimated to <br /> have eroded from the Coal 3asin is : <br /> 547 , 044 , 200 , 000 tons of rock, most of it Cretaceous <br /> and later 1 , 037 , 800 , 000 tons of coal <br /> Total : 548 , 082 , 000 , 000 tons <br /> (Collins , 1977) <br /> I <br />