GC7WKYV I "Scree" Owyhee: A Slippery Slope (Earthcache) in Idaho, United States created by El Nimrod

GC7WKYV ▼

A cache by El Nimrod Message this owner

Hidden : 3/12/2019

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Size: Size: other (other)

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Geocache Description:

This earth cache brings you to one example of several showing scree and talus alongside the road through the Owyhee Canyonlands . The Owyhee Uplands are largely composed of basalt and rhyolite rocks resulting from volcanic activity that began about 17 million years ago and ended about 2 million years ago. The purpose of this earthcache is to examine the formation and characteristics of mountain scree. A detailed discussion of the geology of Owyhee County, not the intent of this earthcache, can be found here for those interested (click)(click).

Talus (pronounced, TAY-lus), or scree as it is sometimes referred to, is defined as a pile of rock fragments lying below a cliff face, which were derived from the physical and mass weathering of the cliff face. Typically, talus lies along a steep slope as in the example shown below, and in the one you are visiting today in the Owyhee Uplands. The size of the scree in the talus deposits can range from grains of sand to large boulders.

* Sand is the smallest grain of rock found in talus deposits.
Diameter <~1 mm (0.04 inches)
* Soil is larger than sand but still relatively small
Diameter ~1 mm to ~10 mm (0.04 to 0.4 inches)
* Regolith can be relatively large fragments of rock or small rock like sand and soil.
Diameter ~1 mm to ~0.5 m (0.04 to 19.5 inches)
* Large rocks are the largest material found in talus deposits. They usually consist of boulder-sized fragments (as long as the boulder is not loose, it is not considered regolith).
Diameter >~0.5 m (19.5 inches)

The relative age of talus compared to the surrounding rocks can be estimated by using geologic context. For example, scree in talus deposits are derived from an adjacent cliff, so the scree must be the same age as the cliff the scree came from. Then, using the geologic principle of superposition, we know the rock underlying the talus must be older than the talus deposit itself. In the case of the talus deposits in the Owyhee Uplands area, the talus deposits are of Jurassic and Cretaceous origin. The talus deposits are of the same age as the top cliff layer of the hill. The rocks underlying the talus deposits are of Triassic or Permian age.

Talus Formation: Weathering, Erosion & Mass Wasting

Rocks or scree talus deposits are derived from the adjacent cliff. Screes form as a result of a process called weathering (click). In weathering, rocks near the surface of the earth (at the top of the mountain) disintegrate, and are transported away down slope (called mass wasting) from their original placement thanks to the eroding forces of wind, water, and ice.

There are two main types of weathering:

Chemical weathering (click) happens when, due to exposure to certain surface conditions, original materials transform into substances which are different in both physical characteristics and composition. As a result of this change, the ‘newer’ substance is more susceptible to the physical effects of erosion. Some types of rock are more affected by agents of chemical wearing than others.

Mechanical weathering(click) happens when rocks get broken apart into smaller particles as a result of physical processes. An example of a physical process which causes mechanical weathering is frost. When frost forms on the surface of rock, it significantly increases the volume of the surface. This, in turn, causes increased outward pressure on the rock surface and can result in the breaking of rocks.

In the cooler months, the cliff is weathered away when rainwater seeps into pores and cracks of the exposed rocks of the cliff. The rainwater then freezes, expanding its volume by 9% when it becomes ice. Since the volume of the rainwater is now greater than when it was liquid, the pores and cracks are expanded by the growing ice. The expansion of the pores and cracks of the exposed rock allows more rainwater to seep in, and eventually, the expanding ice will shatter the exposed rock ("frost wedging or shattering"), adding scree to the pile of talus. The presence of vegetation (trees) and their expanding root systems also promote cracks and fissuring. The top of the cliff face retreats and the cycle repeats season after season.

The predominant processes that degrade a rock slope depend largely on the regional climate (temperatures, amount of rainfall and snowmelt, etc) and irregularities of the cliff's rocky face. Historically, the Owyhee River Canyonlands receives an annual average precipitation of ~ 43 cm (17 inches) and average nightly low temperatures below freezing for the months of October to April (inclusive), ideal conditions for scree formation.

Angle of Repose

The angle at which the talus is resting is referred to as the angle of repose. When granular material is poured onto a flat surface, a pile will form that is roughly conical in shape. The internal angle between the flat surface and the surface of the pile is known as the angle of repose. Many factors can affect the angle of repose: density, surface area and shape of the particles, and the material's coefficient of friction. Material demonstrating a low angle of repose, such as a pile of sugar or salt on a tabletop, forms flatter piles than material with a high angle of repose, such as the rocks forming the talus slope you are studying here today.

Logging Requirements:

Please send the answers to the following required questions to me through my geocaching profile before logging as found. DO NOT post any answers on your log, encrypted or otherwise.

(1) List the name GC7WKYVJ I "Scree" Owyhee in the first line of your email. Also list the number and name(s) of people in your group.

(2) Describe the appearance of the scree slope from this vantage point (i.e., Is the scree made up of large or small rocks? Is the scree of equal size all over the slope or is there a mixture? Presence of vegetation?)

( 3) By what mechanism (type of weathering) do you think this scree field was formed? What factor(s) led to this conclusion? 

(4) Estimate the relative angle of repose of the scree slope

(5) Differentiate between weathering and erosion.

(6) (Optional) Per current www.geocaching.com guidelines, photos are no longer required. However, they are encouraged since they can help clarify that you have visited the location if your other logging requirement answers are vague. Post a picture of yourself and your GPSr with your log that shows the scree field in the background.

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I "Scree" Owyhee: A Slippery Slope EarthCache

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