The Front Range, of which this park preserves a choice sample, was buckled in the fashion of a great long wrinkle in a carpet. This “roll” of rock was about 200 miles long and some 40 miles across. In its earlier stages it was covered by the arched-up sediments, but, as time passed and erosion continued, the inner core of earlier crystalline rocks was exposed once again. Today, all traces of the former thick mantle of sedimentary beds are gone from the park. They are still present beneath the plains to the east and the basins to the west, and the cut-off ends of some of them now lie exposed in a tilted position against both east and west flanks of the mountains. The sandstones of some of the hogback ridges crossed by the approach roads from Lyons and Loveland are a part of this once continuous overburden. Uplift continued intermittently during many millions of years. In the western section of the park, volcanic eruptions took place. Specimen Mountain is the remnant of a volcano; some of its flows are seen today as the cliffs behind Iceberg Lake, on Trail Ridge Road. Great sheets of lava and other volcanic rocks piled in layers now make up much of the Never Summer Range. Eventually, these rocks, too, will be stripped away by the relentless work of erosion; this will require millions of years. An unusual feature of the landscape here is the rolling, sometimes flattened character of many mountain summits. Trail Ridge Road crosses several miles of one of these summits—a gently rolling upland above 11,000 feet. These mountaintops appear to be all that is left of an old land surface that once may have been continuous far eastward over the area occupied today by the Great Plains. Such surfaces, of which the mountains in the park show many good remnants, are called peneplains. Their presence atop the mountains is a part of the evidence suggesting that the range had been worn down by erosion to a fairly flat upland a few million years ago. Then renewed uplifting occurred, and streams draining the highland gradually cut canyons two or three thousand feet into the elevated surface. THE WORK OF GLACIERS These canyons were filled by glaciers at intervals during the million years of the ice age. This period saw the formation of vast ice fields over much of northern North America. The causes of the ice age are complex, but its effects on our landscape are marked and convincing. [Illustration: Sandstone hogbacks flank the mountains on the east. Scene near mouth of Big Thompson Canyon, west of Loveland.] [Illustration: Remnants of an erosion surface on peaks south of Trail Ridge Road.] [Illustration: High mountain lakes are set among the scars of glacier excavation. Arrowhead Lake, in Gorge Lakes Canyon.] [Illustration: Great forests, high peaks, and cool summers add to the appeal of Rocky Mountain National Park.]