OUTLINE OF MAJOR POINTS
The rocks of the two regions range in age from Precambrian to Recent. Most were deposited in shallow seas. In addition, there are extensive deposits of continental origin of several different kinds. The thickest units belong to the Belt series of Precambrian age. Study of these has contributed to revision of concepts as to the character and stratigraphy of the rocks of the series. The principal contribution, obtained through areal geologic mapping over a broad region, is the evidence of marked lateral changes within short distances. In the regions represented in plates 1 and 2 the exposed part of the Belt series is now regarded as consisting, in ascending order, of the Ravalli group, Piegan group, and Missoula group. Each of the groups comprises subordinate units but subdivision is incomplete at present.
Sufficient new information in regard to the stromatolite content of the rocks has been obtained to produce new concepts as to the conditions during deposition of the Belt strata and to justify the hope that intensive study would yield additional valuable data applicable to problems in ecology and stratigraphy. Rezak (1957) has developed new names for the stromatolites, which make many of the names previously in use obsolete.
Paleozoic rocks are exposed only in areas where intense deformation has taken place, so that many of the details of their stratigraphy were not worked out and the rocks are mapped in systemic rather than formational units. The formations are broadly similar to those known to be present farther south but minor differences exist. The Cambrian system is represented only by scattered exposures, in most of which several of its formational units are not exposed. All the Ordovician and Silurian, part of the Devonian, and all the Permian systems are believed to be unrepresented.
No Triassic rocks were recognized. The Ellis group (Jurassic) is present but in most places is represented by narrow fault slivers. Slightly consolidated sedimentary rocks of Tertiary age line the sides of the major valleys and form erosional remnants in the plains close to the mountain border. They are extensively masked with Quaternary deposits, from which they could be discriminated satisfactorily only by detailed work. Pleistocene and Recent glacial and fluviatile deposits floor most of the mountain valleys and cover broad expanses on the plains.
The only intrusive igneous rocks are sills, dikes, and irregular masses, most of which were originally gabbro or diabase, now much altered. These have been found only in rocks of the Belt series, principally in the Siyeh limestone. They are themselves of Precambrian age and are thought to be genetically related to the Purcell basalt, flows of which are intercalated in the upper part of the Belt series.
Last Updated: 08-Jul-2008