Dells of the Big Sioux River EarthCache
Dells of the Big Sioux River
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In order to count this Earthcache as a find, you must complete the following tasks and email the answers to me.
1. Estimate the height of the quartzite outcrop at ground zero.
2. Describe the rock in terms of color, formation, and approximate hardness.
3. What would account for the grooves and striations in the quartzite?
4. Why do the tops of the quartzite outcroppings appear to be smooth and polished?
This Earthcache is located at the Dells Scenic Overlook, which offers views of the exposed Sioux quartzite and the Big Sioux River. There is a shelter and picnic table nearby, so bring your camera and a picnic lunch and enjoy the view!
The Dells, which gives the town “Dell Rapids” its name, is a gorge on the Big Sioux River in southeastern South Dakota. In some places, the gorge is eighty feet deep. This area is noted for its scenic beauty--in particular for its unique Sioux quartzite formations. Geologists have classified the rock formations in the area as part of the Potsdam Formation--an outcrop of Sioux quartzite that extends at least 2600 feet deep at the Dell Rapids quarry site.
The origins of Sioux quartzite date back 1.8 billion years ago in the slow and gentle currents of the first inland streams and seas. This rock began as grains of sand that water worked and sifted for a long period of time before depositing on the shore of an ancient shallow sea. Over millions of years, layer upon layer of quartzite grains grew, became compressed, and endured heat created by the sheer weight of their own burden. The compression and heat transformed the internal structure of the rock from a soft, sedimentary stone to an extremely hard and durable quartzite that, in some places, is hundreds of feet thick.
One billion years ago, Sioux quartzite was exposed over a wide area that stretched from just west of the Missouri River in central South Dakota to the bend in the Minnesota River. (Note: those rivers did not exist one billion years ago.) Throughout the course of millions of years, later inland seas added their deposits of chalk, limestone, and sandstone over large sections of this massive quartzite formation. As glaciers advanced and retreated during the Pleistocene era (geological time period marked by repeated glaciation), this large quartzite ridge was buried under tons of glacial debris. Today, only a tiny fraction of the quartzite is exposed.
Sioux quartzite exists beneath twenty-five counties in eastern and central South Dakota, eleven in southwestern Minnesota, two in northwestern Nebraska, and at least one in northeastern Nebraska.
Thousands of years ago, a shift in the Earth’s surface at this location caused an enormous fissure, or crack, in the quartzite ridge. A huge flood during late glacial time plucked blocks of quartzite out of the ridge. Over time, these cliffs were formed by strong currents of water passing over exposed Sioux quartzite fissures through a process called erosion. Erosion is a natural process that moves solids, such as sediments, soil, and rocks, from one place to another. Usually this transportation occurs by wind, water, or ice.
NOT A LOGGING REQUIREMENT: Feel free to post pictures of your group at the area or the area itself - I love looking at the pictures.
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