The area's land ownership is divided between Bureau of Land 

 Management, Red Rock Lakes National Wildlife Refuge, Beaverhead 

 National Forest, Agricultural Research Service, State of Montana 

 and private. 



Throughout this report, "Centennial Valley" will be used to 

 refer to the entire study area, which encompasses the Montana 

 side of the Centennial Mountains, and the lower foothills of the 

 Gravelly and Snowcrest Ranges. 



Ownership within the study area is depicted on Figure 1, 

 based on BLM digitized data, as updated by Department of State 

 Lands in post-1982 corrections. The 1990 Southwestern Montana 

 Interagency Map produced by the BLM was used to resolve 

 disparities between the two preceding maps, unless map 

 consultants responsible for these products directed otherwise. 



Geology 



The Centennial Mountains belong to the Laramide orogeny 

 system and are included in the broader category of "Central 

 Rockies" (Eardley 1951) . The mountains are tectonic in origin, 

 arising from the prominent "Centennial Fault"; a fault block of 

 east-west orientation that runs along the northern foot slope of 

 the mountains. The Centennial Mountains to the south of the 

 fault are the uplifted range-block and the Centennial Valley to 

 the north is the down-thrown basin block, resulting in a 3,000 

 foot fault scarp. The uplift of the Centennial Range began in 

 Lower Cretaceous time, which marked the advent of the Laramide 

 orogeny, and continued to early Eocene time (Honkala 1960) . 



The Centennial escarpment was further modified by glaciation 

 during the Pinedale and Bull Lake stages (Taylor and Ashley 

 1990) . Small terminal moraines at the foot slope of the 

 mountains are common as are small north-south trending glacial 

 valleys and cirques. More recently, the scarp was modified by 

 avalanches and landslides which are evident along the mountain 

 sides. 



The rocks comprising the Centennial Mountains are: 1. 

 Precambrian metamorphics; 2. Paleozoic, Mesozoic, and Cenozoic 

 sediments; and 3. Cenozoic volcanics. The watershed to the east 

 which includes the Alaska Basin area and the Tom Creek drainage 

 is comprised almost entirely of Precambrian metamorphosed 

 carbonates (Taylor and Ashley 1990) . 



Above the Precambrian metamorphics, at the headwater of Red 

 Rock Creek and Tom Creek, sedimentary rocks prevail. Represented 

 in these high altitude rocks are Cambrian rocks which are 

 progressively overlain by sedimentaries of Devonian Jefferson 

 Limestone or Three Forks Formation; Mississippian Madison 

 Formation Limestone; Permian Phosphoria Formation' Pennsylvanian 

 Amsden and Quadrant Formation; and in some areas. Tertiary 

 volcanics (Egbert 1960, Mann 1960, Taylor and Ashley 1990). With 

 the exception of the Phosphoria Formation which contains cherts, 

 shales, and phosphorite (Cressman and Swanson 1960), the above 

 strata are primarily composed of limestone, dolomite, and 

 sandstones (Mann 1960) . 



The continuity of the Centennial Mountains is disrupted by a 

 north-south trending fault through the Odell Creek drainage. 

 Honkala (1960) theorized that this is a high-angle normal fault. 

 The area west of the fault is composed of Tertiary volcanics. 

 Mount Baldy dominates the area at an elevation of 9889 feet. 



