Wednesday, October 16, 2013


The State of Colorado is blessed to have a variety of scenic features associated with mountain glaciers.  These glaciers, once large, slow-moving rivers of ice flowing “down slope” in response to gravity and high pressure, occupied mountain valleys across most of the State’s high mountain ranges during the Pleistocene Epoch.  The Pleistocene begin perhaps as early as ~2.6 Ma and ended ~11.7 ka.  The glaciers, some many hundreds of feet thick, ebbed and flowed throughout the Pleistocene but geologists have determined that the two most recent glacial peaks were approximately 200 ka –130 ka  (Bull Lake Glaciation) and about 110 ka-11.7 ka (Pinedale Glaciation).  Matthews (2009) noted that in Colorado mountains the glacial peaks were around 130 ka --150 ka and 20 ka with most local glaciers having receded/melted by about 12 ka (Matthews 2003).

During these periods of mountain glaciations, larger continental glaciers covered parts of Canada and the northeastern United States.  The Pinedale Glaciation (western mountains) is roughly equivalent to part of the Wisconsin Glaciation (Northeast/Midwest) where ice sheets reached as far south as the State of Wisconsin.  The Bull Lake Glaciation (western mountains) is about the same as part the Illinoian Glaciation in the Northeast/Midwest where continental ice sheets reached as far south as southern Illinois.  Previous continental glacial events in the Northeast/Midwest, referred to as the Kansan and Nebraskan, are difficult to identify in the mountains of Colorado.

It also is important to note that all of these dates and correlations are approximate and seem to constantly change with new and better dating techniques. In fact, at any one time concurrent geological literature might list several different beginning and ending dates for any specific glacial event.  However, all geologists seem to be working together to tie events into a time scale anchored by marine oxygen-isotope stages (Pierce, 2003).

During my student days many eons ago the Quaternary seemed about the easiest geological period to understand---no dotted lines and names easy to understand (and pronounce).  However, there have been tremendous advances in our understanding of geological time and events during the last half-century.  And, these new advances show no evidence of slowing down.  It is an exciting time to study geology.   

One of the many results of Pleistocene glaciation is the formation of landforms that give our mountains their striking beauty.  Perhaps the best-known location in Colorado to observe landforms resulting from Pleistocene glaciation is in Rocky Mountain National Park.  Here visitors can observe, from high lookouts, a living wonderland portrayed for both the professional glaciologist and the causal interested observer.  Numerous publications and websites describing the geology are readily available (for example see:

Although I recognize the majestic beauty of the Park, I have come to appreciate greatly the glacial topography of other Colorado mountains, especially the Mosquito, Sawatch, Collegiate, Sangre de Cristo and Ten Mile ranges.  One can easily hike or drive to many of the features, and the crowds are few and far between.  Often on my hikes I see no other person on the trails.

Horseshoe Cirque in Mosquito Range.
One of the most recognizable glacial features in Colorado, Horseshoe Cirque, is located in the Mosquito Range west of Fairplay.  There are two major reasons the Cirque is so unique: 1) the glacier carved the Cirque in sedimentary rocks; 2) the sedimentary rocks are dipping steeply to the east.  Most high mountain ranges (Horseshoe Mountain has an elevation of 13,898 feet) in Colorado have igneous or metamorphic rocks at their summits.  These “hard” rocks are usually Precambrian in age, or were emplaced during the Laramide Orogeny. In both instances, the sedimentary rocks have been eroded away and are usually absent; Horseshoe Cirque is an anomaly.  In addition, igneous and metamorphic rocks are usually massive and continuous sharp bedding planes may not be readily evident.  At Horseshoe Cirque the upper walls expose the Mississippian Leadville Limestone (marine rocks) and a few Laramide igneous sills (magma intruded parallel to the bedding planes) (McGookey, 2002).  The lower Cirque walls expose the Cambrian Sawatch Sandstone (marine shoreline/beach deposit) and older Precambrian granites.  In all instances the bedding planes are prominent and the dip of the beds is quite evident.

A beautiful day at 13,889 feet on summit of Horseshoe Cirque.  Frost-shattered Leadville Limestone forms base rock.
The Laramide Orogeny is the mountain building event most closely associated with the current ranges in Colorado.  Beginning in the late Cretaceous (about 80 Ma), and continuing for perhaps 40 my into the Tertiary, the Laramide was responsible for forming most faulted and folded ranges and accompanying basins in Colorado.  Near Fairplay, the Mosquito Range was uplifted as part of the larger Sawatch Uplift (the Rio Grande Rift, now holding the upper Arkansas River Valley, separated the Mosquito Range from the Sawatch Range in the mid Tertiary) and most exposures of the sedimentary cover were eroded away with the sediments being transported into the nearby basins.  An exception was the sedimentary section at Horseshoe Cirque.

Looking east, down-stream from the Cirque, at a glacial U-shaped valley.  Tarn in foreground.
Leading away, and trending east from the Cirque, is a tributary to Fourmile Creek; the streams flowing in a typical U-shaped glacial valley.  Near the headwall of the Cirque is a small lake called Leavick Tarn (tarn is the geological term for a cirque lake).  Additional unnamed lakes are also present near the threshold of the Cirque.  Also of interest are the glacial erratics scattered in the morrainal material down valley from the Cirque.  These erratics, mostly pieces of Leadville Limestone, were dropped by the receding glacier.

Glacial erratics in hummocky ground morraine debris.   
The Leadville Limestone, the major host rock for precious metals west of Horseshoe Cirque near the town of Leadville, also has been mined near the Cirque and adjacent Peerless Mountain and small glory holes and exploration pits are scattered all over the area.  Most of the mining occurred post-1870 after Samuel MacMillian discovered silver near the head of Fourmile Creek.  In 1880 the Mudsill Mine was opened on the north rim of Horseshoe Cirque (McConnell, 1966).  Mining must have continued well into the 20th century as “modern looking” equipment is scattered about at several localities.

Glory hole in Leadville Limestone.

Scattered remains of mining on Finback Ridge.

Although Horseshoe Cirque may be observed at a distance while traveling US 285 south of Fairplay, the Cirque is best seen in a close-up view.  Travel US 285 for two miles south from Fairplay and turn right (west) on Park County 9 (Fourmile Creek Road and access to Mt. Sherman).  Depending upon your vehicle (PU’s/SUV’s OK), travel west on PC 9 for about 11 miles, passing the mill at the ghost town of Leavick, and turn left on a 4-wheel drive road for about two miles.  Horseshoe Cirque then looms directly ahead.  You may want to walk up the road to Finback Ridge and hike the north rim of the Cirque to the summit of Horseshoe Mountain.  At 13,898 feet it is among the 100 highest peaks in Colorado.

May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view. May your mountains rise into and above the clouds.
Edward Abbey


Matthews, V. and contributing authors, 2009, Messages in Stone, Colorado’s Colorful Geology 2nd Edition: Denver, Colorado Geological Survey.

McConnell, V., 1966, Bayou Salado, the Story of South Park. Denver, Sage Books.

McGookey, D. P., 2002. Geologic Wonders of South Park, Colorado: Midland, TX, Donald P. McGookey, 2002.

Pierce, K. L., 2003, Pleistocene Glaciations in the Rocky Mountains: Developments in Quaternary Science 1 (available at ).