Tuesday, May 21, 2013


Landscape map of Utah.  Courtesy of Utah Geological Survey.  SLS: Salt Lake City; SRS: San Rafael Swell; B & R: Basin and Range; T: Transition Zone.
I recently returned home from Salt Lake City after attending the Rocky Mountain Federation of Mineralogical Societies annual show and meeting hosted by the Wasatch Gem Society.  In doing so, I also had the opportunity to explore some of the local geology---although the presence of snow kept me off some of my favorite roads and trails.  If readers plan on exploring Salt Lake City and environs, I would suggest the purchase (may need to be used book sites) of two really fine books:  Utah’s Spectacular Geology, 2005, Lehi F. Hintze; and 2) Geological History of Utah, 2009, Lehi F. Hintze & Bart Kowallis.

In the road trip, I drove via US 40 west from Colorado and traveled directly south of the Uinta Range.  These mountains, containing the highest peaks in the state (in excess of 13k feet), owe their origin to the Laramide Orogeny (Cretaceous—Eocene), and are part of the Middle Rocky Mountains Physiographic Province.  The Uintas are an east-west, fault-bounded anticline that shed sediments off to related basins to the north (greater Green River basin) and south (greater Uinta basin).  The core of the mountains consists almost entirely of the Uinta Mountain Group, a late Proterozoic (late Precambrian) quartz sandstone that is a red to pink color.  This hue is probably due to oxidation of hematite found in the rock.  The age and color of the unit strongly resembles similar rocks exposed in Glacier National Park and Grand Canyon National Park.

Gates of Ladore where the Green River begins its journey through the Uinta Mountains.  Rocks of the Precambrian Uinta Mountain Group crop out within the Canyon.  Photo courtesy of USGS and was taken in 1871 during the 2nd Powell expedition. (see blog posting of March 13, 2013). 
Mining activities associated with metals are almost non-existent in the mountains (lack of igneous intrusions) and therefore “exciting” mineral specimens generally are unavailable. Perhaps the most spectacular geological feature associated with the range is the evidence left behind by Pleistocene glaciation in the form of cirques and u-shaped valleys.  Many airlines heading to Salt Lake City from the east often fly along the crest of the mountains and window seats provide spectacular views.  Auto travelers wanting a closer look at the range have their choice of two north-south roads trending through the mountains.  UT 191 heads north from Vernal to the Flaming Gorge area while near the east end of the range UT 150, known as the Mirror Lake Road, runs from Kamas north to Evanston, Wyoming.  I consider this latter road the most spectacular mountain road in the state; unfortunately, snow did not allow my travel this year.

The Wasatch Range is the major north-south trending range in Utah and runs from the Idaho border to mid-state where it “transforms” into the Wasatch Plateau that in turn continues as part of the Colorado Plateau-Great Basin Transition Zone southwest to the state border (see map).  The mountains are the westernmost part of the Middle Rocky Mountains and the Wasatch Fault (west side) marks the beginning of the Great Basin.

Unlike the Uinta Mountains, the Wasatch Range has a variety of rock types covering  many different ages from the Precambrian to the Pleistocene.  There also are numerous mineralized areas generally related to Cenozoic igneous intrusions; many are close to Salt Lake City and some seem OK for collecting but determining land ownership is a tricky situation.  I always consider one or more of the following: 1) read Jim Wilson’s great book, Rock, Mineral and Fossil Localities of Utah; 2) check ownership and collecting issues with the local office of US Forest Service; 3) visit at a “rock shop”---I have found people at Rockpick Legends in Salt Lake City particularly friendly and informative. 

The Wasatch Range at Salt Lake City is impressive and I always appreciate the high peaks as well as the numerous major canyons cutting into the range.  In arriving from the east I followed I-80 traversing through Parley’s Canyon and again was duly impressed with both the canyon mouth as well as the western view to the basins and mountains.  Immediately north of this canyon is Emigration Canyon, the route of early LDS settlers into the valley. To the south is Mill Creek Canyon that actually terminates within the mountains (but has a road).  Further south are the Cottonwood Canyons, two canyons that are perhaps the most spectacular of the mountain accesses and will take travelers to modern boom “towns” that now house large ski resorts; however, at one time the canyons hosted major mining areas.  The best known of the old mining camps is probably Alta (up Little Cottonwood) where several dumps usually offer collecting possibilities.  However, this trip snow covered past collecting areas.

A hike up to Bell’s Canyon produces a fantastic view.  The mountain glacier that carved the canyon did not reach the valley floor and left behind terminal and lateral moraines.
What I appreciate most about these two canyons is their great display of geology, beginning with the entrances.  Big Cottonwood Canyon (UT 190) starts out following Big Cottonwood Creek in a V-Shaped canyon, one that was created by stream erosion/cutting.  However, about half- way up, the valley widens out into a nice U-shaped canyon indicating the presence of a Pleistocene mountain glacier (but one that did not reach the end of the valley).  The road terminates at the ski resort of Brighton but a partially paved road leads over Guardsman Pass into the Park City.

The glacier in Big Cottonwood Canyon reached about half-way down and deposited a terminal moraine—with a nice U-shaped canyon extending upstream to Brighton.
Immediately south of Big Cottonwood is Little Cottonwood Canyon containing the road (UT 210) to the old mining town of Alta with its numerous mine dumps.  Little Cottonwood has a U-shape throughout its entire length indicating the glacier actually extended beyond the mountains.  Near the entrance of the canyon is the Temple Quarry where giant blocks of quartz monzonite were taken out and transported downtown to construct the LDS Temple.

Mouth of Little Cottonwood Canyon showing the characteristic U-shape of the glacially scoured canyon. 
The Utah Geological Survey has a nice PDF with information about the geology of the Cottonwood Canyons at: http://geology.utah.gov/online/pi/pi-87.pdf
Salt Lake Valley itself has quite conspicuous geological features that most locals call benches and actually are ancient shorelines of Pleistocene Lake Bonneville.  The Lake Bonneville Basin covers a large portion of western Utah but also extends into eastern Nevada and southern Idaho—perhaps on the order of ~20,000 sq. mi.  Although Lake Bonneville is a late Pleistocene event, freshwater lakes have been present, off and on, in the Basin for the last 1-2 million years (perhaps longer) with Great Salt Lake being the current incarnation.  But it was Lake Bonneville that created these shoreline sand and gravel deposits with the highest bench being the Bonneville Level about 5100 feet in elevation and easily observable on the western flanks of the Wasatch Range.  The Provo Level, at about 4740 feet, is very noticeable as the foundation for many buildings at the University of Utah.  Lake Bonneville had a complex history of “ups and downs” attributed to catastrophic overflows near Red Rock Pass in southern Idaho and climate changes.  By perhaps 12 ka Lake Bonneville was “finished” as a large lake.  The Great Salt Lake is a small remnant of Lake Bonneville and is “salty” since it does not have an outlet.

Near Big Cottonwood Canyon. The horizontal terrace is the Bonneville shore line. Below it is a very visible fault scarp indicating movement along the Wasatch Fault within the last few thousand years.  Photo courtesy of USGS and snapped in 1901.
I spent several years studying the large mammals that inhabited the shoreline environments of Lake Bonneville and the record is quite impressive: Musk Oxen, Bison, Mammoths, Bears, Mastodons, Mountain Sheep, Horses, Wolves, Foxes, and others. The Utah Geological Survey (www.geology.utah.gov) has several publications and presentations on both Lake Bonneville and the Pleistocene fossils.

Across the valley to the west is a dominant topographic feature, the Oquirrh  Mountains.  Within these mountains, and highly visible, is the Bingham Canyon Mine, one of the largest open-pit mines in the world and in production since 1906.  The mine has a footprint of nearly 2000 acres, is perhaps 3200 feet deep, and 2.5 miles wide.  The mine managers, Kennecott Utah Copper Corporation, also operate a concentration plant, a smelter, and a refinery. The mining is from a porphyry copper deposit with a very low grade copper ore of disseminated grains and coatings of minerals in a quartz monzonite (granite type rock).  I wanted to visit the mine overlook; however, a giant rockslide (perhaps the largest in a made-made structure in history) recently closed any visitor viewing of the pit.

The open pit copper mine at Bingham Canyon.  Photo courtesy of Kennecott Copper and Rio Tinto.
As students at the University we were always told that all of the visible http://l.yimg.com/g/images/spaceout.gifrocks in the Oquirrh Mountains, with the exception of the igneous stock, belong to a single unit---the Permian-Pennsylvanian Oquirrh Group.  That in itself is interesting, but to make it more so, geologists now know that this big pile of sandstone, limestone, and shale originated perhaps 50-60 miles to the west and was transported to the present location by a low-angled thrust fault!

The Salt Lake City valley is situated in the Basin and Range since the great fault fronting the Wasatch Range is the beginning of the Province. From this fault west to the Sierra Nevada Mountains is an area unique to the country.  Geographers often call it the Great Basin since all of the drainage is internal—there are no streams that externally drain the area.  Geologists prefer Basin and Range, a term indicating numerous uplifted and fault-bounded mountains (horsts) separated by down-dropped valleys (grabens).  Unlike the mighty ranges of the Rocky Mountains (compressional tectonics), the Basin and Range is related to late Tertiary crustal stretching (extensional tectonics).  In addition, many of the ranges have late Tertiary intrusions and outpourings of extrusive igneous rocks.  This igneous event is responsible for several deposits of metallic minerals associated with boom towns (Gold Hill, Tintic, Ophir, etc.) and other mining activities, and offer a wealth of opportunities for collectors.  I have visited and collected at many of these in past trips.  For example, see postings: A Pretty Green Rock (12-8-11); Wendover Will and Adamite (4-23-12); Azurite at Gold Hill (4-26-12); What is That Green Stuff ? "Medmontite" (8-2-12); Notch Peak: Great geology (6-2-11).

I always enjoy the short rides heading east up the Cottonwood Canyons to the ski resorts and old mines of Brighton (in a nice glacial cirque), and Alta.  There are geological signs along the road indicating the formations and the scenery is spectacular.  However, I was unable to take my usual walk around the cirque lake near Brighton, and the road to Park City over Guardsman Pass was closed. The Utah Geological Survey has a great travel road log describing a trip from Salt Lake City up Parleys Canyon to Park City, over Guardsman Pass and back down Big Cottonwood Canyon:  http://geology.utah.gov/online/pdf/pi-09.pdf.  

Near the mouth of Little Cottonwood Canyon is the G. K. Gilbert Geologic View Park described at: http://geology.utah.gov/surveynotes/geosights/gilbertpark.htm.

I also squeezed in a walk downtown as the Survey has a download describing the building stones: http://geology.utah.gov/geo_guides/slc_bldg_stones/index.htm.

The Great Bar at Stockton, Utah, as described by G. K. Gilbert.  Photo courtesy of US Geological Survey (Monograph 1, 1890).

And finally (penultimate), I hopped in my car and headed west on I-80 to get a closer look at Great Salt Lake and the Bonneville Salt Flats.  What I wanted to do is revisit a site that G. K. Gilbert described as “The Great Bar at Stockton, Utah.”  This is a fantastic land form, a really large lake sandbar or actually a series of bars and spits, associated with Lake Bonneville (rather than an adult watering hole).  I found the site by heading south on UT 36 south from the town of Tooele for perhaps 5 miles and then turning west on a gravel road. 

I also hustled north of Salt Lake City to the “causeway” (UT 127), a road stretching from the main land (city of Syracuse) west to Antelope Island, the largest island in the lake.  The island holds a state park, 500 free-ranging bison (plus other animals and birds), and some of the oldest Precambrian rocks in Utah. 

It was a busy trip, but an interesting one.  The Show was great and I even picked up a couple of specimens for my collection.  The trip home was uneventful, except for the snow at Park City, and the traffic through Denver!

ark City!