Prairie, Peak, and Plateau: A Guide to the Geology of Colorado

by John Chronic; Halka Chronic

Front Range

The easternmost range of the Rocky Mountains is the longest continuous uplift in the state. It is a relatively simple faulted [anticline] extending from Canon City northward to the Wyoming border, where it splits into two ridges, the Medicine Bow Mountains and the Laramie Range.

Longs Peak challenges technical climbers with its 2000-foot vertical east face, the Diamond. This magnificent cliff is the result of glacial action and freezing and thawing in homogeneous but fractured [granite]. The small remnant of ice and snow at the lower left is all that remains of the [glacier]. The flat summit may be part of an ancient erosion surface formed toward the end of Precambrian time. (Jack Rathbone photo)

Along the highest portion of the range, from Pikes Peak to Rocky Mountain National Park, the Paleozoic and Mesozoic sediments formerly draped over the top of the range have long since been washed away, leaving only the [gneiss], [granite], and [schist] of the mountain core. The almost flat tops of Longs Peak, Mt. Evans, and Pikes Peak, and the rolling upland traversed by Trail Ridge Road in Rocky Mountain National Park are thought to be remnants of the 600-million-year-old erosion surface that once existed at the top of the Precambrian rocks, and that still exists below the [sedimentary rocks] of the Plains Province. This surface, formed near sea level, has been raised 12,000 to 14,000 feet within the Mountain Province.

Throughout most of its length, the Front Range displays some of the most striking high-altitude scenery in the world. Particularly accessible areas, well worthy of visits, are Rocky Mountain National Park, Berthoud and Loveland Passes, Mt. Evans, and Pikes Peak. In these areas the Precambrian rocks can be seen and studied, and the effects of [glaciation] observed.

The [granite], [gneiss], and [schist] of the mountain core are shattered and broken into blocks of various sizes. The breaks between the blocks are called [joints] if there is no apparent displacement between adjacent blocks, and [faults] where there is obvious displacement. The joints frequently appear in parallel arrays or sets; there may be two or more intersecting sets, giving a cross-hatched appearance to large exposures.

East-west profile across Rocky Mountain National Park, through Grand Lake and Longs Peak, showing the inferred position of the original surface of the anticlinal uplift of the Front Range. This diagram is generalized, and [faults] are not shown. (USGS Bull. 730a)

Restoration of surface which emerged from Cretaceous sea Restoration of Dakota sandstone MIDDLE PARK Grand Lake Longs Peak Foothills GREAT PLAINS [Sedimentary rocks] [Granite] and [schist] Sedimentary rock of plains South Platte R.

Big Thompson Canyon, west of Loveland on U.S. highway 34, is carved in almost vertical layers of Precambrian metamorphic rocks. Gently dipping Late Paleozoic and Mesozoic [sedimentary rocks] of the Fountain, Lyons, Lykins, and Morrison Formations can be seen in the distance, capped by the Cretaceous Dakota Sandstone. (Floyd Walters photo)

The Precambrian rocks vary from place to place. Several irregular masses of [granite], called [batholiths], make up portions of the range. Batholiths are large intrusions of molten rock that cooled slowly at great depth. The minerals in them form distinct crystals, often quite large. The Pikes Peak Granite and the Boulder Creek Granite are examples. Highly contorted and banded [gneiss] and [schist] are well exposed elsewhere, particularly in the Idaho Springs-Central City-Black Hawk region.

Along the flanks of the Front Range, the eroded edges of the [sedimentary rocks] which once covered the range are exposed. These rocks are usually tilted sharply against the mountains, as at Garden of the Gods, Denver’s Red Rocks Park, and the Flatirons near Boulder. The Rocky Mountain Association of Geologists has erected a plaque explaining the geology of the Red Rocks area; look for it about half a mile northeast of the Red Rocks Amphitheater. Tilted layers of Paleozoic and Mesozoic sandstones form [hogback] ridges along the mountain front, and stand out clearly on aerial photographs.

In some areas, particularly near Boulder, Coal Creek, and Golden, the tilting of the sedimentary layers has been so extreme that the layers are upside down. [Basement] rocks may even be thrust out above them.

Sandstones and [conglomerates] of the Pennsylvanian Fountain Formation [dip] steeply toward the plains along the eastern edge of the Rockies. Near Denver, erosion has carved these rocks into a natural amphitheater, now the site of Red Rocks Amphitheater. Precambrian [granite] forms the hill in the background. (Jack Rathbone photo)

Further north, near Loveland and Lyons, as well as further south at Colorado Springs, irregularities in the uplift have caused abrupt breaks in the generally smooth eastern edge of the range. [Folds] and [faults] in these areas trend northwest, cutting across and offsetting the mountain front.

South of Colorado Springs, between Fort Carson and the NORAD installation in Cheyenne Mountain, Mesozoic rocks are faulted against the mountain front. Paleozoic rocks are deeply covered by as much as 3000 feet of Mesozoic sediments. They come to the surface about 10 miles further south.

RAMPART RANGE Garden of the Gods Ute Pass [Fault] MANITOU SPRINGS PIKES PEAK [MASSIF] CHEYENNE MOUNTAIN COLORADO SPRINGS CROSS SECTION Ute Pass Fault Rampart Fault Tertiary Mesozoic Paleozoic Precambrian

West of Boulder, several intersecting sets of [joints] pattern the Precambrian rocks above Boulder Creek. (John Chronic photo)

The west margin of the Front Range is not as sharply defined as the eastern margin. Prominent [faults] edge North, Middle, and South Parks, however. The northern end of the range merges with the Medicine Bow Mountains, where [dips] of sedimentary rocks seldom exceed 30 to 40 degrees. At its southern end, the Front Range plunges into the plains, although a southwest-trending ridge connects it with the Wet Mountains.

Within the Precambrian core of the Front Range, many economic mineral deposits have been found. These are discussed in [Chapter III]. Glacial features of the Front Range are discussed in Chapter II in the section on the [Quaternary Period].