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.