Rocks of Gitchie Manitou
In Iowa, United States
Size:  (not chosen)
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The Rocks of Gitchie Manitou Earthcache description:
In the languages of many Indian tribes, the name "Manitou" refers to a good spirit, often inseparable from their concept of nature. So they called this place Gitchie Manitou, a Siouxan name for "Great Spirit", or "Great Force of nature." The latter could not be more appropriate.
Tucked away in the extreme reaches of northwest Iowa and bounded on two sides by South Dakota, Gitchie Manitou is a world quite its own. It is a world of wonder, of strange plants, strange rocks and strange history. Though small in size, for it encompasses only an isolated patch of rock, woods and prairie, its many wonders might be expected to draw visitors from far and wide. But an hour's drive in any direction reveals that few other than near by residents have ever visited this living natural museum.
Gitchie Manitou is perhaps most noted for its ancient rock formations and has been designated a geological preserve by the State Preserves Advisory Board. Sioux quartzite is an unusually resistant pink colored rock. Outcroppings here date to the Precambrian Era and are estimated to be no less than 1.2 billion years old as determined by the Uranium-Lead dating method. The Uranium-Lead dating method uses the time of the decay of Uranium-235 to Lead-207 to determine the age of material that underlie the thick sequence of sedimentary rocks in parts of the state.
The beginnings of the quartzite go back to when prehistoric seas receded leaving sandstone composed of rounded, quartz sand grains and a small amount of an iron-containing mineral called hematite. This sandstone was subjected to incredible stress by the earth's mountain building forces. Under this stress the sand particles became cemented together by quartz. In the course of time a solidly interlocking mass of crystals was formed and the sandstone was metamorphosed into quartzite. Since the sand grains are also of the mineral quartz there are no minute openings in quartzite as there are in sandstone giving the Sioux quartzite its unusually resistant quality. The hematite is the cause of the red or purplish color of the rock.
Evidence suggests that a large mountain range once dominated this now gentle landscape, stretching from eastern South Dakota, across Iowa's northwest tip, as far east as present day New Ulm, Minnesota. Over the ages these mountains gradually eroded away, and the quartzite was mostly buried. Finally, great glaciers of the relatively recent ice ages passed over Gitchie Manitou, buckling and tilting its rocks until they lay exposed much as they do today. Wind and weather polished the exposed rock to a glossy sheen.
The quartzite sticks up through soil and is exposed along river valleys in many places in southeastern South Dakota and southwestern Minnesota, as well as in Gitchie Manitou Park. The formation is known to underline an area of about 6,000 square miles in the three states. The falls from which Sioux Falls was named is over quartzite. Fragments of this purplish quartzite are to be found in the subsoil and along the streams in Iowa and elsewhere in the Midwest as moved along through glacial action.
At Gitchie Manitou Park the rock outcrops irregularly in the form of a low ridge, shining and beautiful in the sun. The surface is partly covered with gray lichens. Careful examination of the stone will in places reveal the ripple marks left the ancient oceans. Circular and crescent-shaped marks may also be noted. These marks may have been made by glacial action. More likely, however, they were made by a large river of glacial melt water which once flowed down the Big Sioux Valley. Rocks carried along by this swift current banged against the rock surface, Producing circular cracks. Outcrops just outside the park to the east have smoothed and scratched surfaces made by the glaciers which passed over the region during the Ice Age.
The rock also has many straight cracks. Some of these are the cracks between the beds of quartzite, and are known as bedding planes. Others cut across the beds and are known as joints. There are two prominent sets of joints, at right angles to each other. These cracks help in the weathering and erosion of the quartzite.
Some History and Fauna Notes
Awed by the rock formations aside their sacred river Tetonkaha, now called the Big Sioux, Indians of several tribes revered this place as holy. Legends say warring groups would often lay down their weapons and enter Gitchie Manitou in harmony to worship the Great Spirit.
Giving a thoroughly western flavor to the preserve is the presence of a large community of prickly pear cactus, which spreads across the exposed quartzite, root systems nurtured in cracks and crevices of stone. This is believed to be one of the largest concentrations of prickly pear found in the state. Early June visitors thrill to the delicate beauty of sulfur-toned cactus flowers.
To log this Earthcache, please post a picture of the rocks with you or your GPS in the photo and answer the following question:
At the coordinates is a large feature. In an e-mail to the cache owner, please identify the feature and the cause of the feature.
This Earthcache description was developed from the following sources:
Rock Formation of Gitchie Manitou Park, Iowa Conservationist 10(2):
113&120, Gwynne, Charles S., 1951
Big Sioux Float, Iowa Conservationist 31(10): 3-5, 16, Gibson, Jon, 1972
A Great Force of Nature, Iowa Conservationist 39(9): 2-3, Harr, Doug,
Iowa's Geological Preserves, Iowa Geology 1984(9): 16-19, Prior, Jean
Geologic Timekeepers, Iowa Geology 1985(10): 18-21, Libra, Robert D.,
Landscape Features of Iowa, Iowa Geology 1995(20): 14-21, Prior, Jean
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Last Updated: on 6/24/2015 7:49:56 AM Pacific Daylight Time (2:49 PM GMT)
Coordinates are in the WGS84 datum