Teton fault and it's resulting geological  lift

Grand Teton Geology

Grizzly Bear, Grand Tetons
Grizzly Bear, Grand Tetons

As Ansel Adams said: “The grand lift of the Tetons is more than a mechanistic fold and faulting of the earth’s crust; it becomes a primal gesture of the earth beneath a greater sky.”

I prefer Ansel Adam’s “photographer speak” too “geologist speak” so I have done what I can to make geologic science intelligible because I also have a hard time deciphering what geologists say while speaking over our heads.

The Teton Range is the focal point of the Grand Teton Park. What made the Teton Range famous is its, in your face, precipitous drop from the summits of its panoply of canine tooth shaped mountain summits to the valley floor below. Unlike most mountain ranges, this precipice lacks foothills, or lower peaks, which obscures the views of high peaks as in most ranges. Access to the spines of most mountain ranges requires a trip through the foothills first, but not here, and this is how it happened.

Upthrown Fault Block
Upthrown Fault Block

To understand geologic process you must understand rocks. Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments of material. Together, all these particles are called sediment. Gradually, the sediment accumulates in layers and over eons it hardens into rock.  Metamorphic rocks are formed when they morph from one state to another because of the intense heat and pressure of the earth. The rocks that result from these processes often have ribbon like layers and may have shiny crystals, formed by minerals growing slowly on their surface over time. Igneous rocks are formed when magma (molten rock deep within the earth) cools and hardens. All are present here in the Grand Tetons. A dike or dyke in geology is a sheet of rock that formed in a crack in a preexisting rock body. However, when the crack is between the layers in a layered rock, it is called a sill, not a dike. Clear as mud huh?

Mount Moran geology
Mount Moran geology

The bedrock of the Teton Range is primarily ancient, hard crystalline rock and darker-colored metamorphic rocks. The geological processes that led to the current composition of the oldest rocks in the Teton Range began about 2.5 billion years ago when sand and volcanic debris settled into an ancient ocean. Then deep below the ocean floor heat and pressure worked its geological magic and the sediment metamorphosed into gneiss (pronounced “nice”). White layers, rich in quartz and feldspar, interspersed with dark layers rich in biotite mica and hornblende, produce this zebra-striped rock. Gneiss is a hard rock that has a mineral composition similar to granite as they both consist of feldspar, mica, and quartz. Later magma forced its way up through the cracks in the gneiss to form layers of granite compositionally similar to gnist but formed by volcanic process instead of metamorphosis. The light-colored stripes slicing across the gneiss are veins or dikes of igneous granite. Although the minerals in granite are the same as gneiss, granite is speckled because of its molten origin instead of layered, as is the metamorphic gneiss.  Silly me, I had previously assumed the whole escarpment was granite.

Granite example, it also shows how bolders are strewn about when a glacier melts and leaves a moraine
Granite example, it also shows how bolders are strewn about when a glacier melts and leaves a moraine

Metamorphic gneiss and igneous granite were subsequently intruded by later Precambrian dark-colored igneous rock dikes around 1.3 billion years ago. The best exposure of these dikes is on the eastern face of Mt. Moran where the "Black Dike" extends from the summit of the peak downward to Leigh Lake on the floor of the valley. It’s hard to miss the 200-foot wide vertical Black Dike that transects the face of Mount Moran. This igneous dike is made of diabase, an iron-rich magma squeezed into cracks and cooled. These are some of the oldest rocks in North America.

The ancient seas retreated then between fifty-five, and eighty million years ago tectonic forces began to lift the Rocky Mountains out of the former seabed. Another 60 million years would pass before tectonic plate activity triggered the birth and subsequent growth of the Teton Range.

Gneiss example
Gneiss example

About ten million years ago, this region began to stretch and the Earth's crust cracked forming faults. The Teton Fault is ours; it extends 40 miles along the base of the Teton Range forming a fine foundation for hundreds of Jackson Hole hotel rooms. As the ground breaks, the Teton Fault’s two crustal blocks slip vertically past one another generating an earthquake, each time the crust broke; an earthquake averaging magnitude 7.5 sends the mountains skyward while dropping the valley floor. These seismic events average one per millennia. Rates of movement on the fault have changed throughout time. The Tetons gained most its height in the last 5 million years, and likely accelerated during the last 2 million years when the Yellowstone Volcano at the northern end of the Teton Range had its three, world changing eruptions.At the end of the Pinedale Glaciation 14,000 to 16,000 years ago, melting of the Yellowstone ice cap and deglaciation of the Teton Range, action on the fault increased; this was likely resulting from changing stresses on the fault as glaciers melted.

Glaciation Graphic
Glaciation Graphic
View from Teton Valley Idaho
View from Teton Valley Idaho

This vertical movement on the fault has caused the dramatic topography of the Teton Range.The east slope of the Teton Range rises sharply, from 5,000 to 7,000 feet above the valley floor. The view is most dramatic from the east; on the west side, the Teton Range rises more gradually and does have foothills which obscure much of the escarpment. One million years ago the Huckleberry Ridge Tuff was created deposited along the west slope of the north part of the range. A tuff is a light, porous rock formed by consolidation of volcanic ash.

Earthquakes have built the Grand Tetons, but it has been glaciers that have given them much of their unique character. The Tetons have been glaciated at least three times, with the oldest event being the most significant one. The ice sheet in many places in Jackson Hole exceeded 2,000 feet in thickness, and later glacial events eroded or covered parts of earlier ones. During the latest glaciations, ice flowed down canyons in the Teton Range onto the floor of Jackson Hole and built the moraines that dam Jackson, Leigh, Jenny, Bradley, Taggart, and Phelps Lakes. There are still eight small glaciers in the Grand Tetons, but these are not remnants of the Pleistocene Ice Age, but formed during a cold period called the Little Ice Age that started around 1and ended around 1850s, but for now we are fortunate to have these souvenirs of the Little Ice Age. It has been quite a process, and I am happy with the results.

Grand Teton geology features
Grand Teton geology features
Custom Search
Rocky Mountain Rotors - Yellowstone Helicopter Charter Rocky Mountain Rotors
Advertise with the Greater Yellowstone Resource Guide
Grand Teton Photography and Field Guide
Buy Now
jumping trout