Horizon's Gift of Glaciers EarthCache

This cache has been archived.

Geocaching HQ Admin: It has now been over 30 days since Geocaching HQ submitted the disabled log below and, unfortunately, the cache owner has not posted an Owner maintenance log and re-enabled this geocache. As a result, we are now archiving this cache page.

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Hidden : 11/17/2006
Difficulty:
1 out of 5
Terrain:
1 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

Scattered across Horizon Middle School's rich prairie grasslands, are large granite boulders. Where did they come from?

The land around Horizon Middle School in Bismarck, North Dakota, has all the features needed to learn about the geology of the region. The loamy soils of the Upper Great Plains support a broad variety of medium grass prairy species. On a cloudy, drizzly day, we traveled to the newest middle school in Bismarck. I knew that the school was built on the top of a draw that leads eventually down to the Missouri River, on the north edge of town and thought there could be some good geographical features to look at. I was not disappointed. As we pulled up into the parking lot and proceeded to the west, it didn’t take long to find a characteristic of the glaciated prairy. On the side of upper reaches of the drainage area there is a large boulder. This rock is very normal for areas that were formed as the earths crust cooled from liquid. This type of rock is known as an igneous rock. Look at the map of North America and find the area that is made extensively of igneous rocks.
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As I entered the field, I noticed that the grass is supported by a rich, black soil. It’s obvious why they call this region, the “bread basket” of the United States. On looking closer, the soil is made of fine particles that are neither smooth like clay nor gritty like sand. This is typical of a mix of fine sand particles mixed with some clay but also many generations native prairie grasses that have lived and died and mixed in with these other components. The black soil is a good indicator of old plant life in the process of decay to feed the upcoming generations of plant life.

While this may seem insignificant, this black colored soil keeps large shares of carbon from entering the atmosphere as carbon dioxide. This is the same gas that is the culprit for much of the global warming debate that has been in the news for the past several years. This process has been going on for some time but has been increasing for the last one hundred years. These soils have formed from long periods of erosion from the Rocky Mountains to the west. This area was once a large inland ocean that was slowly filled up with sediments and has been raised out of the ocean since Jurassic times. In this slow process of lifting, layers of ocean life, fine dust, and sands eroding out of the mountains have been mixed thoroughly together. During this process of change the areas were once a low swampland that became the source of much of the coal of that lies in areas from these bluffs westward. As climates changed and the region received more limited rainfalls, generations upon generations of prairie grasses lived and died to enrich these soils.

More recently, as this area of the Earth has experienced cold cycles, large glaciers proceeded across Canada and the northern United States stopping at it’s furthest extent just past Bismarck bringing with it materials carried from far away. If North Dakota is roughly 450 miles wide, how far would you estimate that these rocks needed to have traveled from the igneous rock region of North America? As you look across the prairies, you will see evidence of many various sized intruders from the north carried down by the slow, relentless march of glacial ice from times past. Many different occupants of the North American continent have found these rocks useful in a number of ways.

To claim this Earthcache, while standing at the cache:

1. Look around and so that you will be able to login and list the different ways that people have used these granite boulders.

2. Estimate the size of the boulder in cubic meters.
(As an extra challenge, using the density of granite as 267 kg per cubic meter, estimate the mass of this large erratic.)

3. Look around the area for several of these erratics, and make a list of all the possible colors that these type of rocks can contain. Just look at the larger boulders. Log the colors and enter the type of mineral that is responsible for that color. (There should be 4 standout regions on any one of these rocks)

References: Geology of the Lewis and Clark Trail in North Dakota, by John Hoganson and Edward Murphy.

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