Bedrock Geology Information

Today millions of tons of high grade lead-silver ore probably lies hidden beneath the surface at Alta. Many of the largest ore bodies at Alta were cut-off by high angle faults, and the faulted-off portions of these deposits were never found. Somewhere in the archives of old mining companies are clues to where these deposits might be found. But don’t be too concerned about mining threatening the environment of Little Cottonwood Canyon. Alta is not destined to returned to its mining routes. Federal and State land use regulations insure that skiing, hiking and fishing are the preferred land uses for Little Cottonwood Canyon.

The most dramatic visible effect on mining in Little Cottonwood Canyon was the removal of all the trees. A fire that started in downtown Alta in 1878 further decimated the tree population. Although mining is generally blamed for the logging of the canyon, the harvest of logs for houses in the Salt Lake Valley began long before silver was discovered. Trees began to grow again in the canyon after the turn of the century, but todays forest is largely the result of the replanting of evergreen seedlings upon the recommendation of Alf Engen and under the direction of the U. S. Forest Service beginning in the 1930s. This is continued by Alta Ski Lifts Company.

The rock surrounding the ore bodies at Alta is largely composed of limestone, which neutralizes acid generated by pyrite and other minerals in the ore bodies. Because of this, ground water at Alta is essentially pure with little or no metallic contaminants (think about that before you buy bottled water at Alfs).

Mining and mineral development has had lasting effects on Little Cottonwood Canyon. The railroad to Alta was constructed in 1875. The line was built from the mouth of the canyon to Tanners Flat in about two months, accelerated by the availability of Chinese laborers left idle by completion of the transcontinental railroad at Promontory Point in 1869, and the complete absence of environmental regulations or employee safety laws. The current road follows much of the rail line.

Although millions of tons of ore were mined from Alta, today there is almost no evidence of this mining activity. Most of the ore at Alta was high grade silver-bearing galena (lead sulfide) with a relatively small proportion of lower grade material. The residue of mining was largely removed by the massive floods of spring. It all went down the creek.

Between 1867 and 1940 the Big and Little Cottonwood mining districts produced:

Thats pretty small potatoes when compared with the Mother Lode of California or the Comstock Lode of Nevada; nevertheless, Alta had its impact.

As the Alta Stock began to cool an enormous volume of super heated water developed within and near the top of the magma body. This hydrothermal fluid transported lead, silver and other minerals, depositing them in fractures and receptive rock layers in the sedimentary rocks, forming the bonanza lead-silver deposits of the Little Cottonwood Canyon Mining District

Right around the Alta Stock the heat was so intense that elements of the limestones were reorganized and recrystallized into new skarn minerals. Many new minerals were formed in this environment. Fine-grained bands of garnet are visible in outcrops in Grizzly Gulch and along the summer trail to Catherines Pass.

The invasion of the Alta Stock had a dramatic affect on the enclosing sediments. The heat of the stock recrystallized much of the limestone, forming tan to white marble. Marble forms the sandy soil near the end of the traverse into Catherines Area and much of the cliff-forming outcrops beneath the Supreme Chair and at the Hellgate Cliffs.

The older sedimentary rocks were intruded (invaded) by a molten mass of magma deep in the earths crust approximately 35 million years ago. These rocks, called the Alta Stock, have the classic granitic salt and pepper texture and are compositionally a granodiorite, an igneous rock similar to granite but with more calcium, iron and magnesium.

Overlying the Ophir Shale are a series of limestones spanning an enormous period of geologic time. These rocks are major cliff formers at Alta, forming Devils Castle, Supreme Bowl, much of Catherines Area, Westward Ho, Collins Face, High Rustler, Eagles Nest and Hellgate Cliffs. These limestones were formed in shallow, crystal clear water in the tropics, a good image to keep in mind on a windy winter day on the Sugarloaf chair.

Overlying the Tintic Quartzite is the Ophir Shale a dark red-brown, deep water sediment. The Ophir Shale is best known at Alta for forming the jagged outcrops above the High Traverse. This rock unit crosses the trail near the end of the High Traverse and when snow depths are low, fragments of these outcrops litter the snow. The Ophir Shale also forms the cliffs on the upper part of the Back Side. Eddies High Nowhere, Gunsight and the High Notch are narrow chutes cut through this rock unit. The Ophir Shale also runs along the base of Devils Castle but is difficult to see because outcrops are largely obscured by talus from overlying rocks.

The bulk of the rocks at Alta, the rocks that we ski on, record the advances and retreats of ancient oceans that occupied this area of North America between 540 and 330 million years ago (Paleozoic Era). The oldest of these rocks is the Tintic quartzite a hard, dense, buff-colored rock that generally forms rounded and rolling terrain. The Tintic Quartzite underlies Mount Baldy and Sugarloaf Peak. It is also broadly exposed beneath the Sugarloaf chair lift, Ballroom and in much of Collins Gulch. The erosional characteristics of the Tintic Quartzite are responsible for much of the intermediate terrain at Alta.

The oldest rocks you will see at Alta form the dramatic outcrops on the southeast flank of Mount Superior on the north side of Little Cottonwood Canyon. These are extremely old rocks (Upper Proterozoic Era, + 800 million years). The lighter brown rocks to the west consist of the Little Cottonwood Formation. The dark brown rocks to the east are the Little Cottonwood Tillite formed by an ancient glacial event. You can see folded beds in the Tillite near the summit of Mount Superior formed by a mountain building event that preceded the formation of the Wasatch Mountains by hundreds of millions of years.

The bedrock geology of Alta and the surrounding area was mapped by the U. S. Geological Survey in 1943.