Sedimentary Sorting EarthCache
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This cache is located on a trail that leads from the parking area to the river. This path leads down a wash through a slot canyon until you get to the river. There are a lot of interesting geological sites along the hike.
All along your hike you will see different layers of earth that have been deposited and then tilted. At this point on your hike there is a large layer of sedimentary rock known as conglomerate. Conglomerate rocks are sedimentary rocks that are made up of large sediments like sand and pebbles. The sediment is so large that pressure alone cannot hold the rock together; it is also cemented together with dissolved minerals. These sedimentary rock layers were deposited layer after layer. There are about 11 different distinct deposition layers in this formation.
In a conglomerate rock there are usually different sized grains in the layering. In this case, these different sized grains are sorted. Sorting refers to the uniformity of grain size in a sediment or sedimentary rock. Particles become sorted on the basis of density. High energy (high velocity) currents can carry larger fragments. As the energy or velocity decreases, heavier particles are deposited and lighter fragments continue to be transported. This results in sorting due to density. If the particles have the same density, such as all grains of quartz, then the heavier particles will also be larger, so the sorting will take place on the basis of size. We can classify this size sorting on a relative basis: well-sorted to poorly-sorted.
This formation is a well sorted area that well call graded bedding. It was most likely formed in a river area. As river current velocity decreased, the larger or denser particles are deposited first, followed by smaller particles. This results in bedding showing a decrease in grain size from the bottom of the bed to the top of the bed. This gives us a method for determining tops and bottoms of beds, since reverse grading will not be expected unless deposition occurs under unusual circumstances. Each time that the deposition of a new layer happens (11 times), you will see the larger grains at the bottom and the grain size will decrease as you move up.
The larger grains have a specific shape. They could be either jagged or smooth according to how they were transported. The amount of time spent in the river will allow the grains to either weather off and become rounded, or they are still jagged (not been in the river).
The best place to start this cache is from the parking area. It can be found at N 34 26.622 W 114 15.892 Follow the wash to Sarah’s Crack. Once past the crack, you will continue down the wash until you get to the coordinates.
To Log this Cache: If you log this cache without sending an email to the owner with the three questions answered your log will be deleted. PLEASE make sure you do not post the answers to the following questions on the web page, instead they are to be emailed to the owner for verification.
1) Make sure you bring a ruler (mm or cm)- What are the grain sizes (in centimeters or millimeters) of the largest (bottom) part of each deposit? What are the grain sizes of the middle level of the deposit? What are the grain sizes of the smallest part of each deposit?
2) Are the grains in the conglomerate rock rounded or jagged? Explain why they are that way.
3) This is not the only location that you can find this happening in the area. Because there is a tilt to the land and all the uneven weathering, there are other locations that have this same graded bedding occurring. Send me the coordinates of another location along the wash.
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