Cascade Canyon and Jenny Lake EarthCache
Hidden : 9/26/2010
Difficulty:
2 out of 5
Terrain:
1.5 out of 5

Size: Size: other (other)

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Geocache Description:


Few landscapes in the world are as striking and as memorable as that of Grand Teton National Park. Rising abruptly from the valley floor, the Tetons offer a testament to the power and complexity of nature.



Geologic History :

The Tetons are among the youngest ranges in the Rocky Mountains. Although there was structural uplift of an ancestral Teton Range during Paleocene time (about 50 million years ago), the modern range is the product of uplift along the Teton Fault which began 9 million years ago. The Teton Fault, which parallels the eastern front of the Teton Range from north to south, is an eastward-dipping fracture in the earth's crust along which movement has occurred. The block on the western side of the fault (called the footwall because it lies beneath the inclined surface of the fault) was uplifted, while the block on the eastern side of the fault (called the hanging wall because it lies above the inclined surface of the fault) moved downward. This sort of motion defines a type of structure called a normal fault. Such faults are the result of extension or stretching in the earth's crust.

The amount of uplift, or structural relief, can be determined by comparing the elevation of a geologically recognizable horizon on opposite sides of the fault. The Precambrian-Cambrian unconformity atop Mt. Moran is at an elevation of about 12,500 feet, while the same boundary is buried far beneath the surface of Jackson's Hole, at an elevation of 22,500 feet below sea level. The amount of structural relief is 35,000 feet, which indicates a rate of uplift of 4.5 inches per hundred years.

Geology :

The bedrock of the Teton Range is primarily ancient, hard crystalline rock including darker-colored metamorphic rocks and lighter-colored igneous rocks. These rocks were formed during the Archean portion of Precambrian time, prior to 2.5 billion years ago, and were subsequently intruded by later Precambrian (Proterozoic) dark-colored diabase dikes around 1.1-1.3 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.

The Precambrian crystalline rocks are overlain by a blanket of Paleozoic sedimentary rocks. These rocks are exposed on the western slope of the Teton Range and form "The Wall", a great cliff of stratified rocks which may be seen from the floor of Jackson's Hole as one looks westward up the great U-shaped valleys which transect the range. Most of the sedimentary rocks were deposited as layers of sand, mud and lime in great inland seas whose shorelines fluctuated in position across this region from the Cambrian period at the beginning of the Paleozoic Era (around 550 million years ago) through the Mississippian Period (around 320 million years ago).

Younger Paleozoic and Mesozoic rocks were deposited in the region but were removed by erosion from the western slope of the Tetons following uplift along the Teton Fault. These younger rocks can be seen in the Gros Ventre Mountains which bound Jackson's Hole on the east

The boundary between the Precambrian crystalline rocks and the overlying sedimentary rocks is a major unconformity (or gap) in the rock record. This unconformity is visible from the floor of Jackson's Hole on the summit of Mt. Moran. The flat top of Mt. Moran coincides with this surface of erosion between the Precambrian and younger rocks, and on the summit, a small remnant of the oldest Paleozoic rocks (the Flathead sandstone) is preserved as a light-brown-colored outcrop on the southern half of the peak. The black dike, which traverses the summit of Mt. Moran, does not cut across this sandstone, indicating that the sandstone is younger in age than the dike.

Earthcache:

One of our favorite features of the Park is Jenny Lake. The posted coordinates will take you to a pullout, with parking, along the Jenny Lake road. There you will find a National Parks Information Panel that tells the story of how Cascade Canyon and Jenny Lake were formed. We should note that the Jenny Lake one-way-road is closed to vehicle traffic annually from November 1 trhough April 30.

As you look across the lake you can see Cascade Canyon, the huge U-shaped notch that cuts right through the heart of the Teton Range. The south side of the canyon is dominated by Grand Teton Peak, elevation 13,370 feet. The north side features Mount St. John, elevation 11,430 feet. The floor of the canyon is at about 7,500 feet. The depth from mountain peak to canyon floor is roughly equivalent to the Grand Canyon of the Colorado. Cascade Canyon was originally cut into the mountain as a narrower V-shape by running water. Later, as the climate changed and snow collected, a large glacier formed.

While tectonic movement of the Teton fault lifted the mountain range, erosion sculpted the landscape. Starting two million years ago, massive glaciers, up to 3,500 feet thick, periodically flowed south from Yellowstone and filled the valley – eroding mountains, transporting and depositing huge volumes of rocky glacial debris. As ice sheets filled the valley, alpine glaciers sculpted the jagged Teton Skyline. These glaciers carved the peaks and canyons, and deposited debris along the glacier’s edge.



Logging requirements:

Visit the site at the posted coordinates and read the NPS information panel, then send us an email with "Cascade Canyon and Jenny Lake" and the number of people in your party as the first line. Next, answer the following questions :

1 – What accounts for the broad U-shape of Cascade Canyon?

2 – Jenny Lake was formed approximately 12,000 years ago. How was the depression that impounds the Lake formed?

3 – When was the Jenny Lake area explored and mapped by the US Territorial Survey expedition?

4 – What is the rocky shore material around Jenny Lake called?

Do NOT post answers in your log.
All answers must come from the NPS Information Panel with the same wording.
Googled answers will result in the deletion of your log.

References:
NPS Information Panel
http://en.wikipedia.org/wiki/Jenny_Lake
http://en.wikipedia.org/wiki/Cascade_Canyon
http://en.wikipedia.org/wiki/Geology_of_the_Grand_Teton_area
http://www.winona.edu/geology/travels/tetons/travel.html

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