AW Mountain Geology lesson

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Lesson Objectives

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Ideally, this short lesson should be done on a vantage point so students can really see the effects of glaciation that you are talking about.

Students should be able to identify:

• Key terrain characteristics of the Uinta's and how they were formed by the processes of glaciation
• Definition of key terrain attributes including drainages, basins, aretes, cirques, saddles, and nunataks
• Definition of glacier and how glaciers form
• Moraines- terminal, medial, and lateral
• Rock Types- glacial till, scree, talus, boulders, and felsenmeers

Finally, students will hopefully learn to appreciate how knowing some of the geological events and processes can give them a greater appreciation of the area and assist them as they hike- understanding terrain attributes like drainages, talus fields, and ridge lines.

Key Terrain Attributes

Uinta's are the only prominant east-west range in the lower 48 of the U.S. Timberline is at about 11,000 feet. Divided into the north and south slopes by a prominant ridge line that runs about 60 miles long from east to west. The Uinta's were shaped largely by the advance and retreat of glaciers. During the Pleistocene Epoch (10,000 yrs- 2 mya) the valleys below the peaks were buried repeatedly under accumulations of moving ice. The exposed ridges and peaks, however, stood above the ice. You can tell the difference by looking at the ridgelines. Flat-topped ridges and mountains (liek Bald Mountain) were shaped by a glacier over top of it. Tokewanna peak was not covered and thus remained more pyramidal in shape.

What is a glacier? A glacier is simply acculumated snow. The longer the snow stays in place, the more compacted it becomes. Eventually it turns into ice and "moves" by either advancing as snow acculumates or retreats by snow melt-off. A sidenote: concern today mounts about the melting of glaciers due to global climate change. Glacier National Park in Montana, for example, is projected to have no glaciers left by the year 2025. Historic mountaineering routes in Africa (Kilamanjaro and Mt Kenya) no longer exist as snow caps have almost completely disappeared on these mountains.

Drainage Basins. Drainage basins are formed by water falling down the line of old glacial valleys. The major drainage basins in the Uintas that we hike in include: Lake Fork, Rock Creek, Black Fork, Henry's Fork, and Yellowstone (it helps to have a large overview Uinta map to demonstrate this). All these streams head near the crest of the range in broad amphitheater-like basins, or compound cirques, that were once scoured out by Pleistocene glaciers.

Why is the North slope more dramatic and rugged than the South? Geologists believe that in the last glacial period, younger glaciers remained and carved out the Uinta's. Because north slopes get less sunlight, glaciers can "attack" north slopes with more vigor than they could on south slopes. This caused the more dramatic amphitheaters and "carving" that you see on the north side of the Uintas.

Visible evidence of glaciation in the Uinta's

V-shaped canyon profiles are examples of carving glacial ice as are truncated ridge spurs, falls and cataracts, glacial lakes, and polished rock surfaces. Perhaps the greatest visible examples are the moraines.

There are three types of moraines:

• Lateral- debris that was pushed to the sides of a moving glacier (imagine the snow moved to the side when you shovel a sidewalk).
• Terminal- debris that was pushed down to the end of the glacier
• Medial- debris that was pushed to the middle as two glaciers came together

You can see evidence of glacial caused moraines in many spots in the Uintas including Smiths Fork and Henry's Fork on the North slope. The rock types at moraines (often called "fields" or "slopes") also clue you in to this geological process.

Rock Types at moraines:

• Glacial Till (powdery rocks- very fine)
• Glacial Scree (baseball size and smaller)
• Glacial Talus (refridgerator size to basketball size)
• Glacial boulders (house size to small car size)

Nunataks are peaks, or islands of rock, that rose above the glacial fields. Tokewanna peak is a good example of a nunatak.

Another rock type not formed by glaciers is the felsenmeer. Felsenmeer are scree and talus sized rocks near summits and peaks. These rocks were not formed by moraines but rather by the freeze and thaw cycle in the mountains that splits rocks into increasingly smaller sizes. This is the major erosional process in the Uintas. As water seeps into small cracks and fissures in the rock and subsequently freezes (which it could do 365 days a year at altitude) and thaws, it fractures the rock. Eventually, this process will reduce the mighty Uinta's to piles of rubble!

More information on the Geology of the Uinta's

Major Rock Types of the Uinta’s:

The Uintas are composed mostly of medium grained red or gray quartzite. There is also some thin pebble layers and thick shale layers.

Igneous Rock---The general term for rocks formed by cooling magma. Intrusive igneous rocks were formed by magma that solidified below the earth’s surface. Extrusive igneous rocks were formed be lava that solidified after flowing out above the earth’s surface. When magma cooled slowly it formed large crystals, and the faster the rock cooled, the smaller the crystals, so that rocks that cooled very quickly smooth and glassy with no crystals.

Granite---An intrusive, igneous rock, with coarse, visible crystals. It is the most abundant rock in the Shield.

Greenstone---An igneous rock formed by the cooling of basaltic lava underwater. They are gray-green to yellow-green in color, and are the oldest exposed rocks in most of the Shield.

Sedimentary Rock---the general term for rocks formed by deeply buried sedimentary material compacted into rock by the weight of other material above it. These rocks erode more easily than igneous rocks.

Limestone---a sedimentary rock composed mainly of calcite or aragonite.

Mudstone---a rock made of a mixture of silt and clay.

Sandstone---a sedimentary rock composed of sandy grains.

Shale---a sedimentary rock with a flaky cleavage, such as mica.

Conglomerate—a rock that is actually a conglomeration of many, various sized, pebbles stuck together with sand.

Metamorphic---The term for rocks whose molecular structure is changed by strong, external forces such as heat and pressure. These new minerals and crystals are created from changes in both igneous and sedimentary rocks.

Slate---a fine-grain metamorphic rock.