Geologic Timeline

Geologic time, from the formation of the Earth at ~4.6 billion years ago to the present, is understood and represented by layered rocks throughout the world. By understanding the relative ages of layered and cross-cutting rocks, and the fossils they contain, geologists have developed a geologic time scale. Relative ages are cross-correlated with numerical ages derived from radioactive isotopes of elements contained in some of the geologic units. Using fossils and radiometric ages, geologists can compare the geologic strata of Montana with the "type section" of Devonian rocks exposed in the Devon area of southern England. For example, using fossils, geologists can compare certain geologic strata in Montana with the "type section" of Devonian age rocks in Devon, England and determine that their ages are the same. Radiometric dates tell us that Devonian rocks fall in a range of 369–410 million years ago.

For example, using fossils, geologists can compare certain geologic strata in Montana with the "type section" of Devonian age rocks in Devon, England and determine that their ages are the same. Radiometric dates tell us that Devonian rocks fall in a range of 369–410 million years ago.

The area is structurally complex. Four styles of faulting are present in the northern part of the area. Northwest-striking thrust faults of probable Cretaceous age are the oldest structures recognized. These faults place older rocks over younger rocks, and portions of these structures are steep enough to be considered reverse faults. Normal faults are present behind and

parallel to several of the thrusts, bringing young rocks of the hanging wall down again. These normal faults may merge at depth with the thrust faults. A second type of normal fault is represented by a single low-angle, nearly horizontal structure, with the younger rocks on the upper plate. Age of this structure is uncertain, but most likely early Tertiary. The youngest group of normal faults are steep, with northerly or northeasterly strikes.
Folding in the northern part of the area is typically along north-west trending axes. The carbonate-bearing Wallace Formation was particularly susceptible to folding. The folding probably coincided with formation of the thrust faults during Cretaceous time.
The area with the least amount of structural complexity is the north-central portion of the map along the west side of Petty Mountain and Telephone Butte. A significant section of the Belt Supergroup is exposed here, from the Wallace Formation to the upper part of the Mount Shields Formation.