THE ROCK LAYERS OF THE GRAND CANYON
The top four layers of the Grand Canyon are the Kaibab Formation, Toroweap Formation, Coconino Sandstone and the Hermit Formation. Use the following information along with your own observations at the posted coordinates to answer the required questions for this EarthCache.
This is the top layer of the Grand Canyon. This layer averages about 250 million years old and forms the surface of the Kaibab and Coconino Plateaus. It is composed primarily of a sandy limestone with a layer of sandstone below it. In some places sandstone and shale also exists as its upper layer. The color ranges from cream to a greyish-white. When viewed from the rim this layer resembles a bathtub ring and is commonly referred to as the Canyon's bathtub ring. Fossils that can be found in this layer are brachiopods, coral, mollusks, sea lilies, worms and fish teeth.
This layer averages about 255 million years old and is composed of pretty much the same material as the Kaibab Limestone above. It is darker in color, ranging from yellow to grey, and contains a similar fossil history
This layer averages about 260 million years old and is composed of pure quartz sand, which are basically petrified sand dunes. Wedge-shaped cross bedding can be seen where traverse-type dunes have been petrified. The color of this layer ranges from white to cream colored. No skeletal fossils have yet to be found but numerous invertebrate tracks and fossilized burrows do exist.
This layer averages about 265 million years old and is composed of soft, easily eroded shales which have formed a slope. As the shales erode they undermine the layers sandstone and limestone layers above which causes huge blocks to fall off and into the lower reaches of the Canyon. Many of these blocks end up in the side drainages and down on the Tonto Platform. The color of this layer is a deep, rust-colored red. Fossils to be found in this layer consist of ferns, conifers and other plants, as well as some fossilized tracks of reptiles and amphibians.
GRAND CANYON EROSION
The most powerful force to have an impact on the Grand Canyon is erosion, primarily by water (and ice) and second by wind. Other forces that contributed to the Canyon's formation are the course of the Colorado River itself, vulcanism, continental drift and slight variations in the earths orbit which in turn causes variations in seasons and climate.
Water seems to have had the most impact basically because our planet has lots of it and it is always on the move. Many people cannot understand how water can have such a profound impact considering that the Canyon is basically located in a desert. This is one of the biggest reasons that water has such a big impact here. Because the soil in the Grand Canyon is baked by the sun it tends to become very hard and cannot absorb water when the rains come. When it does rain the water tends to come down in torrents which only adds to the problem. The plants that grow in the Grand Canyon tend to have very shallow root systems so that they can grab as much water as possible on those rare occasions when it does rain. Unfortunately these root systems do nothing to deter erosion by holding the soil in place. Now you've got lots of water, no place for it to go, but down to the Colorado River, and nothing holding the soil and rock in place. The result is frequently a flash flood roaring down a side canyon that can move boulders the size of automobiles, buses and even small houses. This mass that moves down a side canyon during a flash flood is more like fast flowing concrete than water and it can be very dangerous.
After erosion by liquid water the next most powerful force is probably its solid form, ice. In the colder months water seeps into cracks between the rocks. These cracks can be caused by seismic activity, or by the constant soaking and drying of the rocks. When the water freezes it expands and pushes the rocks apart and widens the cracks. Eventually rocks near the rim are pushed off the edge and fall into the side canyons. These rocks sometimes hit other rocks and are stopped but on occasion one fall by a large rock will cause a cascading effect and create a rock fall that will alter the landscape drastically in the side canyon. Debris from rock falls piles up at the bottom of the side canyons and is then carried down to the Colorado River the next time there is a flash flood.
In order to log this EarthCache, send me your answers to the following questions either through email from my profile page.
1: What type of erosion do you think played the biggest role in shaping Duck on a Rock? Please explain your answer.
2: What layer is the Duck on a Rock formation in?
3: Why do you think there is more erosion in the layer Duck on a Rock is in versus the layer immediately below it?
4: Pictures are not required but are appreciated. Please post a pic of you or your GPS from the overlook with the Duck on a Rock in the background.
NATIONAL PARK SERVICE POLICY
In accordance with National Park Service Policy review on "GPS-Based Recreational Activities in National Park areas" updated October 1, 2009, this EarthCache does NOT require or encourage participants to leave the marked trail, move, touch, or in any other manner disturb the natural environment. Please use caution and stay at least 6 feet away from the edge of the canyon at all times. EarthCaches are a type of geocache that may provide participants with a learning experience in the geosciences, and do not involve a physical cache.
This EarthCache was developed in cooperation with and with written permission from the Grand Canyon National Park.