Early Paleozoic geography and sedimentation in the Rocky Mountain Basin were influenced by the positive element, Montania, first noted by Willis, and later described and named by Walcott the “Montana Island.”

A southern Waucobian sea entered the Great Basin and transgressed northeastward over Utah while a northern sea advanced over southwestern Alberta and southeastern British Columbia. Cascadia was united with Laurentia by Montania which formed a barrier 550 miles in width across the Cordilleran geosyncline.

At the beginning of the Albertan the Cordilleran waters invaded the northern and southern parts of Montania from Canada and the Great Basin. The two seas united in northern Idaho and northeastern Washington forming the Idaho strait, and in northern Montana in the Eastern Rocky Mountain Basin. In Middle Cambrian, Montania was a low island whose apex lay in the region of Glacier National Park.

The western part of Montania was elevated before the opening of the Croixan when a sea lay to the east in central-western Montana and extended northwest across southern Alberta into the deeper part of the Cordilleran geosyncline in southeastern British Columbia.

Montania was elevated slightly before the Ordovician and remained above water until Middle or Late Devonian. Consequently, Middle or Upper Devonian limestone rests nearly conformably upon eroded Middle Cambrian dolomite in northwestern Montana.

The climate was warm, humid and uniform throughout the region during most of the Cambrian, but local aridity and possibly lower temperatures may have prevailed during the Croixan.

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