The material carried by the streams during the time of canyon filling was eroded from the mountains. Gradually these mountains were worn away until the summit areas in the Park became a region of rounded hills and ridges, at an altitude of about 8,000 feet, which rose 500 to 1,500 feet above broad valleys that sloped gently east and west to the edge of overlapping stream deposits. To the east the surface probably merged imperceptibly in places with the deeply weathered surface of that ancient plain formed about 40 million years ago and, farther east, with the surface of the stream deposits filling the canyons and extending eastward onto the High Plains. This ancient scenery may be visualized from Trail Ridge (see frontispiece) and many other summit areas, if you look out over the gently sloping uplands and imagine that the deep canyons are not there. The highest hills were where they are today—along the Continental Divide, along Trail Ridge, and in the Mummy Range. The broad intervening valleys were where the canyons are now. The region as a whole, however, was about 4,000 feet lower than today.

The summit uplands, though flat in contrast to the present canyon walls, are not actually flat, as any hiker can testify. Some, such as Bighorn Flats, slope as little as 300 feet per mile; but most slope at about 600 feet per mile, or about 1 inch per step. In places, as between Andrews Pass and Taylor Peak, the slope is 1,000 feet or more per mile. The steepest terrain was probably in the Never Summer Mountains and north of the Park, where the high volcanoes were being destroyed by erosion.

It is logical to ask why the gently sloping uplands might not be actual remains of that deeply weathered 40-million-year-old plain. Proof that the summit uplands in the Park are not a part of that plain can be seen at Iceberg Lake on Trail Ridge Road. There, as we have seen, a volcanic ashflow, only about 28 million years old, fills a former valley in the ancient gneiss. The gently sloping upland extends without interruption across the gneiss, across the tops of the buried valley walls, and across the ashflow rock. The surface of the upland, therefore, must be less than 28 million years old, and not a part of the 40-million-year-old plain. For the last 5 to 7 million years it has remained the roof of the Rockies, enduring with little modification the vicissitudes of subsequent uplifts, faulting, stream erosion, and glacial climate.

Gently sloping upland extending across ashflow rock and gneiss along Trail Ridge Road above Iceberg Lake. (Fig. 13) (Wayne B Alcorn)

About 5 to 7 million years ago, forces within the earth initiated a final broad uplift of the Rocky Mountain region. This was accomplished chiefly by renewed movement along old faults, though in many places opposite to the original movement. Some mountains were uplifted; others collapsed. In like manner, some lowland areas were downdropped and others uplifted. In the Park, the overall effect was to raise the mountains as much as 4,000 to 5,000 feet, which brought the summits to their present altitudes above 12,000 feet. As during previous uplifts, this uplift was uneven and the region was broken by steep fractures, or faults, along which adjacent parts of the mountains were raised or lowered with respect to each other.

As the mountains were jostled up and down, the ability of streams to erode or deposit was greatly changed. Probably some valleys were broken and offset up or down by the movement. Streams formed waterfalls where they flowed across a fault scarp from an uplifted area to a downdropped area. An abrupt change in gradient of canyons in the east flank of the Front Range may have originated in this way. Streams whose courses were blocked by an uplifted mass formed temporary lakes or were forced to turn sharply along the fault to find a new outlet. The great bend in the canyon of Tonahutu Creek at Big Meadows may have such an origin. The displacements tended to be greatest along the flanks of the mountains, although some large displacements occurred within them. However, in many places the offset of adjacent mountain blocks was only a few tens or hundreds of feet. Because Rocky Mountain National Park has not been geologically mapped in detail, the location of such faults and the amount of displacement of mountain blocks adjacent to them are not known.

Movement of the mountain blocks displaced the land surface up or down. Trail Ridge and other parts of the flat-topped upland along the Continental Divide were clearly uplifted. The small, flat summit of Longs Peak appears to have been uplifted some 800 feet higher. The basin of Estes Park may have been downdropped as much as 1,000 feet. Remaining areas of the deeply weathered 40-million-year-old plain were also broken and uplifted or downdropped, so that its original relation to the younger flat-top upland is difficult to determine.

The down-dropped basin of Estes Park. One fault, along which movement took place, lies along the foot of the mountains across the basin. View is northeast from Deer Mountain. (Fig. 14). (Dwight L. Hamilton)

Despite the temporary derangement of the streams, the net effect of the broad uplift of the region was to increase erosion both in the mountains and throughout the regions to east and west. To the north, in Wyoming, the mountains were exhumed; to the south, in central Colorado, deep canyons were cut. In the Park, the shallow valleys extending away from the Continental Divide and the crest of the Mummy Range were deepened, and eventually became winding V-shaped canyons 600 to as much as 1,500 feet deep. The Colorado River to the west, the Cache La Poudre River to the north, and the Big Thompson River to the east all cut deep canyons, in places exhuming former canyons which they had previously filled. Lesser streams stripped away almost all of the older overlapping sediments though some remain in a few places, as at Prairie Divide northwest of Fort Collins. Where the mountains meet the plains, the hogback ridges were again exposed. Even on the plains, rivers such as the South Platte cut broad valleys 600 to 1,000 feet deep. It was a period of great erosion.