Ground-water resources in northeastern Garfield County, Mont., afford a practical and reliable source of stock water on the intermingled public and private grazing lands that together comprise an area of about 1,200 square miles. The oldest formation exposed in the area is the relatively thick and impermeable Bearpaw Shale of Cretaceous .age. Overlying the Bearpaw Shale in succession are the Fox Hills Sandstone and Hell Creek Formation of Cretaceous age, the Fort Union Formation of Tertiary age, and thin glacial deposits .and alluvium of Quaternary age. All but the Bearpaw Shale and the glacial deposits are potential aquifers. Published geologic maps were found to be satisfactory after fitting contacts to the topographic base. Mapping, therefore, was limited mainly to outlining on aerial photographs the alluvial deposits in the stream valleys.
The major structural feature is the Blood Creek syncline, the axis of which plunges eastward 10-15 feet per mile across the southern part of the area. Beds generally dip 15-25 feet per mile toward the synclinal axis. Water in bedrock aquifers is under artesian pressure, .and most wells in Big and Little Dry Creek valleys flow at the land surface.
The only bedrock aquifer having appreciable areal extent is a sandstone 30-70 feet thick that has been mapped by previous investigators as the upper part of the Fox Hills Sandstone. This aquifer crops out in the northern and northwestern parts of the area and dips about 20 feet per mile southeastward beneath younger beds. Most wells in the northern half of the area obtain water from this sandstone at drilling depths of less than 200 feet.
The depth to the Fox Hills Sandstone increases progressively southward, and most wells south of Woody Creek obtain water from irregularly distributed sandstone beds and lenses in the overlying Hell Creek and Fort Union Formations. The depth at which water may be obtained from these beds is not accurately predictable, but the depth seldom exceeds 300 feet.
The results of the investigation indicate that the prospects for obtaining ample water for livestock from wells drilled into the bedrock formations are very favorable in most of the area. The average depth of bedrock wells in the area is 195 feet.
Underflow in the alluvial deposits along all the larger stream valleys also affords a practical source of stock water.
Chemical analyses of samples collected at 43 wells and three springs show the water quality to be generally poor. Water from bedrock aquifer contains 530-5,340 milligrams per liter total dissolved solids, whereas water from alluvium contains less than 1,500 milligrams per liter total dissolved solids. The predominant constituents are sodium, bicarbonate, and sulfate. So far as could be determined, all water supplies in the area are suitable for livestock.