The hypothesis is presented that the thrust faults in the plains adjacent to the Bearpaw Mountains in north-central Montana were produced by a plainsward sliding of the volcanic rocks and underlying strata down the flanks of the low sedimentary arch on which the volcanics rest. It is predicated that this slipping took place on a bed of bentonite in the Colorado shale and was aided by earth tremors that accompanied the last stages of the volcanic outbursts. The flat-lying Upper Cretaceous and lower Tertiary strata in the adjacent plains which, if more rigid, would have obstructed the plainsward sliding of the tilted mountain mass or been overridden by it were shoved plainsward and thrust-faulted for a distance of 20 to 30 miles.
The faults show no progressive plainsward decrease in amount of thrust across the broad area in which they occur and are separated by wide belts of undeformed flat-lying strata. These anomalous features are attributable to slight frictional resistance t o movement offered by a slippery bentonite bed and the probability that the sedimentary rocks over part of the faulted belt were overlain and strengthened by a cover of volcanic rocks that thinned toward the outer margin of the faulted belt. The deformed strata consist chiefly of shales that were too weak to support the load involved in lifting the strata in anticlinal folds. Therefore, thrust faulting, instead of anticlinal folding, took place.