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.
[Illustration: 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.
[Illustration: 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._
[Illustration: 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.
[Illustration: 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.
[Illustration: 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
[Illustration: 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.