The Fallen City EarthCache

MEMORIES AND EARTHCACHE LOG

The earthcache coordinates have brought you to a sign on the side of the road with a convenient parking area to admire the scenery and take a break. You are more than welcome to upload a picture of yourself and your group to your log. This view totally deserves it and it may be a great memory of your travels through the Bighorn Mountains Range.

To log this cache as a valid find please, first read through the earthcache description and then email me the answers to the following questions:

1. Please let me know what type of Wyoming's physiographic category you think this earthcache is located in. In other words on what type of a physiographic category you are standing on admiring the earthcache. Are you on the Bighorn Basin or on the Bighorn Mountains Range? What geological features do make you think you are standing on a basin or on a mountain range?
2. The city felt because of a fault running underneath this area. Can you tell me what type of fault do you think run underneath the Fallen City? Use your GPS to determine your traveling direction and remember that Normal faults run accross and Thrust faults run along the mountain range.
3. When you turned around to look at the rock layers on the opposite bound of the road what two predominant colors can you see on the rock layers? What is the thickest layer main color?
4. What do you think the boulders you are seeing in front of you accross the ravine resemble? (read the sign)
5. Do you have any idea of what material are the boulders made of? What type of rock? (read the sign)

GEOLOGY OF WYOMING

Most of the present big mountain ranges have originated underwater or at least they were at sea level millions of years ago. The Earth's crust, which is some 22 miles thick and composed of dense, crystalline igneous and metamorphic rocks, subsided (sunk) and uplifted (arised) relative to sea level for a long period of time. While Wyoming was below sea level, several thousand feet of relatively flat-lying sedimentary rock accumulated below the oceanic waters or along the shoreline areas.

Throughout a long period of time, there were minor upward and downward oscillations of the crust, but the principal activity in Wyoming was subsidence and accumulation of sediments. Wyoming was last at or near sea level approximately 66 million years ago. Today, most of the sedimentary rock units have been tilted from their originally horizontal positions by large-scale tectonic activity. This activity warped and fractured the crust and the overlying sediments, outlining the mountain ranges and basins and establishing the geologic framework that we see today.

MOUNTAINS IN WYOMING

This episode of mountain building elevated the rocks above sea level and provided the necessary downhill gradients whereby rivers of the region could proceed to dissect the rocks into the existing landforms. The space in between mountain ranges became basins, which at their lowest elevation gather all the water from the mountains in a big river or lake. Later, as these streams cut downward, their courses were locked into the harder underlying rocks and, as a result, such great canyons as the Wind River, the Bighorn, and the Platte evolved.

As a result of this geologic history, Wyoming is now divisible into three major physiographic categories: mountains, the Great Plains of eastern Wyoming, and basins. The landscape and underlying geology of features in each of these categories are very different. In most of Wyoming's mountainous areas, the difference of elevation is in part due to uplift of large segments of the Earth's crust in the form of folds or wrinkles, or blocks bounded by fractures, or a combination of both.

THE FALLEN CITY

The Bighorn Mountains are a more or less continuous chain of mountains that sweep in a great arc (concave to the west) from southern Montana, around the Bighorn Basin, and then westward toward the Owl Creek Mountains. The highest and most rugged part of this arc lies west of the town Buffalo, where the Bighorn Mountain front rises abruptly above the Powder River Basin on the east and culminates in the spectacular summit of Cloud Peak (elevation 13,166 feet).

Several large, prominent fault systems cut the Bighorn Mountains in various places, including the Tensleep fault east of Tensleep, the Big Trails fault in the southern Bighorns, and a series of high-angle reverse and thrust faults along the eastern mountain front. Normal type of faults run accross the mountain range from east to west. While Thrust type of faults run parallel to the mountains range from south to north.

From the eartcache coordinates you should be looking at a marvelous rock formation, which origins can be traced back to a significant fault that runs under this section of the mountain range. At this particular point on the highway you are standing on top of a fault that hundreds of years ago created the rock collapse you are looking from accross the mountain.

THE FALLEN CITY'S GEOLOGICAL TIME PERIOD

Most of the rocks in the Fallen City come from the Paleozoic and Mesozoic eras. As we already know they were uplifted from the bottom about 60 million years ago when this mountains were formed. The evidence of the geological time periods are evident on the different layers of rock that you can see on the side of the mountain. If you turn around for a moment you will notice that in order to build the road, the mountain had to be carved and excavated. It made more evident some of the sedimentary layers that conformed this era of rocks. You can also noticed that along the highway there are several signs that tell you on what layer you are currently driving.