GENERAL DESCRIPTION OF CLIMATE, GEOLOGY, SOILS, AND VEGETATION 



The study area has a typical continental climate having hot summers, 

 cold winters and rapid seasonal transition. At Circle, January temperatures 

 average 13.1°F (-10.5°C) with a mean minimum temperature of 2°F (-16.7°C). 

 Temperatures in July average 70°F (21.1°C) with a mean maximum temperature 

 of 88°F (31.1°C). The frost free season lasts about ninety-nine days 

 (Cordell 1971). 



Precipitation for the area is variable and unevenly distributed and 

 drought strongly influences vegetation. Figure 1 portrays the precip- 

 itation at Circle and Fort Peck for approximately twenty-five years. The 

 fifty-year average at Circle is 12.46 inches (31.65 cm). Between 1952 and 

 1977 the average was 12.9 inches (32.77 cm) with a standard deviation of 

 3.6 inches (9.14 cm). At Fort Peck, the average precipitation between 1955 

 and 1975 was 11.3 inches (28.7 cm) with a standard deviation of 3.4 inches 

 (8.64 cm). 



The annual precipitation of the study area averages between 12 and 16 

 inches (30.5 cm 40.6 cm). The proposed mine area has an average annual 

 precipitation of about 14 inches (35.6 cm) (USDA-DNRC 1977). On the average 

 eighty-two percent of the annual precipitation falls between April and 

 September, and fifty-five percent falls between May and July (Cordell 1971). 



The Circle West study area is underlain by nearly horizontal beds of 

 sedimentary rock which are poorly exposed except in local badlands in the 

 western half of the area. The oldest exposed formation, the Bearpaw Shale, 

 of Late Cretaceous age, is exposed in bluffs along the Missouri River and 

 near Fort Peck. Overlying the Bearpaw are the Fox Hills Sandstone (Cre- 

 faceous), the Hell Creek Formation (sandstone and claystone, some lignite; 

 Late Cretaceous age), and the Fort Union Formation (shale, sandstone, and 

 lignite; Paleocene age). The Fox Hills and Hell Creek Formations have out- 

 crops along the Big Dry Arm of Fort Peck Reservior and along the breaks of 

 'the Missouri River. The Fort Union Formation occupies the central and 

 southern parts of the Circle West study area, and is divided into three 

 members, tlie Tullock, Lebo and Tongue River. The Tongue River Member con- 

 tains the coal beds of the proposed Circle West strip mine. 



During Pleisocene continental glaciation, glaciers deposited till 

 throughout the northern half of the study area. Erosion has mostly removed 

 the till along the main streams and rivers and little remains south of the 

 Missouri River. 



Alluvium consisting of silt, sand and gravel underlies the bottom lands 

 of most of the major streams and rivers in the study area. Saline soils, 

 which result from a combination of poor subsurface drainage and a nearby 

 source of salts, are widespread on poorly drained alluvial terraces. 



