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-17- Over half of the precipitation occurs as snowfall. The remaining precipitation is mostly concentrated in high energy thunderstorm events in late spring and early summer. Some of the runoff flows during the spring and early summer are stored in reservoirs. Runoff during late summer through winter is sustained in perennial streams by the release of stored water from small reservoirs and by discharges from ground water aquifers. Geology The three mines within the general area are all located along the Grand Hogback Monocline. The Grand Hogback forms the steeply dipping eastern edge of the Piceance Basin. The Pennsylvanian throuyh Tertiary aged sedimentary strata which form the Grand Hogback dip 15 to 45 degrees to the west. The dip of the strata decrease southward from 4U to 45 degrees at the Sunlight Mine, to 25 to 34 degrees at North Thompson Creek Mines, to 15 to 2D degrees at the Coal Basin Mines. Several smaller structural folds have developed along the Grand Hogback Monocline as a result of the igneous activity and the tectonic uplift in the region. The Coal Basin Mines are mining the nose of a plunging anticline, the Coal Basin Anticline. This anticline is thought to be related to an igneous laccolith intrusion beneath the southern end of the Grand Hogback Monocline. The Hunter's Point Syncline roughly parallels the Coal Basin Anticline to the north. Both of these structures trend about N45oW and plunges to the northwest. North of these geologic structures, there are the Wolf Creek Anticline and an unnamed Syncline. These structures roughly parallel the strike of the Grand Hogback Monocline. The Wolf Creek Anticline plunges to the north-northwest. The plunge of the unnamed syncline has not been determined. Most faults along the Grand Hogback are perpendicular to the strike of the strata and trend east to west. These are high angle normal faults. Along the Grand Hogback, the amounts of vertical displacement and the amount of faulting increases, from 10's of feet and few faults in the north to 100's of feet and many faults in the south. A fault in Sections 21, 22, 23 and 24 of T9S, R89W separates the Hunter's Point Syncline from the unnamed syncline. This fault separates the area around the Coal Basin Mines and the areas around the other mines into two separate ground water basins. Sedimentary rock units along the Grand Hogback range in age from the Pennsylvanian Age, Eagle Valley Formation to the Tertiary Age, Wasatch Formation. The Eagle Valley Formation is composed of siltstones, shales and evaporite deposits. Due to this unit's low resistance to erosion, the Crystal and Roaring Fork Rivers have established stream valleys along the strike of this formation.