ABSTRACT

INTRODUCTION

The Cordilleran region extends from the east front of the Rocky Mountains to the Pacific Ocean. The present aspect of this province of great mountain ranges, intermontane valleys and plateaus reflects many spasms of crustal deformation, some of which affected areas of earlier deformation. No attempt is made in the present paper to untangle the complex tectonic history of the Cordillera and to pin down the various structures to any particular epoch of deformation. In the Rocky Mountain region, most of the structural units described probably reached essentially their present forms during what is commonly called the Laramide revolution, active from Late Cretaceous into Early Tertiary time. The Laramide revolution was, however, only a major episode in a long continued history of deformation which started in the far west at least as early as the Triassic reached a culmination there in the Nevadan revolution near the end of the Jurassic, and was active in one place or another throughout the Cretaceous. In a general way, belts of intense orogeny travelled eastward with the passage of time, so that effects of the Laramide revolution were far more violent in the Rocky Mountain region than farther west. Although changing in form from time to time and from place to place, this mighty revolution persisted through the Tertiary and maybe still going on.

PHYSIOGRAPHY OF THE CORDILLERAN REGION

The Cordilleran region, largely mountainous, contrasts strikingly with the monotonous Great Plains to the east, but the western Great Plains were affected by deformation as well as the Cordillera because they rises steadily westward to the east front of the Rockies. Starting with elevations above sea level of a few hundred feet in the Mississippi and Missouri Valleys, the surface rises to 2000 feet in South Dakota, Nebraska, Kansas and Oklahoma, and to 4000 feet in places along the western borders of these states. At the foot of the Front Range elevations reach5 000 feet.

This vast region o f the western Great Plains and Rocky Mountains lay beneath the sea during Upper Cretaceous times, about 6 0,000,000 years ago. Since the Rockies attain elevations exceeding 14,000 feet, and formerly were higher, it is evident hat a sizable segment of the earth's crust has been mightily uplifted.

Within the Cordilleran region relief is strong. Great ranges such as the Front Range, Sangre de Cristo, Sawatch, Uinta, Wasatch and Sierra Nevada contrast with Death Valley, partly below sea level, and the Great Valley of California, close to sea level. Much of the Cordilleran region lies at lower elevations than that of the western Great Plains, but one quarter of it lies above 7000 feet. These high areas are distributed from the Front Range on the east to the Sierra Nevada, California Coast Ranges, Klamath Mountains and north-south Cascades on the west; and from the mountains of western Montana, northern Idaho, and northern Washington on the north, to those of central Arizona and southern California on the south.

This content is only available via PDF.
You can access this article if you purchase or spend a download.