Geological Survey Professional Paper 296 Geology of Glacier National Park And the Flathead Region, Northwestern Montana

STRUCTURE

STRUCTURAL HIGHLIGHTS

The rocks in and south of Glacier National Park have been subjected to diastrophic forces at intervals from Precambrian time to the present. Most of the disturbances are reflected only in upwarps, which are so broad and gentle as to be inconspicuous but none the less significant in interpretation of the geologic history. In addition, but more locally, the rocks are intricately folded and broken by faults of several kinds and magnitudes. Igneous activity has left no recognized record since the close of the Precambrian era. The region is best known for its many overthrusts, which formed shortly after the close of the Mesozoic era. The largest, most widely known of these is the Lewis overthrust. This thrust did not, as has been supposed, emerge at the surface close to the border of the present park. Closely spaced, in part asymmetric, folds are associated with the overthrusts. The folding began considerably before the major thrusting took place. Steep faults associated with topographic depressions are major, much discussed features of northwestern Montana and adjacent regions. Features of this kind are present just west of Glacier National Park. Like the more famous ones still farther west, the faults are so poorly exposed that their character and origin are obscure. The simplest and most probable explanation of the features here mapped is that the valleys are graben-like and bordered by normal faults. However, zones of complex folding close to some of the faults suggest that the simple explanation may not give the complete story.

In the pages following pertinent information in regard to each of the structural features is summarized, with a minimum of theory and inference. The descriptive section is followed by one devoted to interpretation of the phenomena.