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- Timeline
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Explore Montana's Geology
Montana's diverse and challenging geology is evident, from its western mountains
lifted and folded by tectonics and sculpted 
by
glaciers, to its vast eastern plains partly overlain by glacial till and
dissected by wind and water.
Using MBMG’s geologic maps, you can study the State’s geologic features in your home or office. Or, you can take a map along while you experience Montana’s geology first hand.
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.
These
map segments are from the Montana portion of the Missoula West
Quadrangle (MBMG
373 ). This and other geologic maps are available
from our 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.  |
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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.
