My next post about the Geology of the National Parks Through Pictures is from our move across the country from Utah to New York. Along the way we visited 13 National Parks as well as some other sites. This was the 12th National Park along the way.
You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.
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Obligatory entrance sign
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| Great Bear Mound |
Created by ancestral people known as the Effigy Moundbuilders, the mounds served several different purposes. Primarily they were used for human burial purposes by piling soil, sod, and clay, along with other materials, in the shapes of animals over the bodies of the deceased. These shapes include birds, bears, water spirits, deer, turtles, beaver, buffalo, canines, and other animals, however the shapes are often difficult to identify along when viewed at ground level. The Lidar image below gives an excellent example of an aerial view of the mounds, with each of these marching bear mounds reaching around 20 meters in length.
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| Lidar of the Marching Bear Group in the southern portion of the park. Image courtesy of the NPS via Wikimedia. |
Found throughout the region, most of the mounds within the park are conical in design. However mounds, like the Great Bear Mound, form the shape of animals (pictured from ground level above). There are also linear and compound designs found within the park, but in far fewer numbers.
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| Types of mounds found at Effigy Mounds NM. Image courtesy of National Park Planner. |
The construction of at least some of the mounds began by first removing the sod or leafy humus from the forest floor. A square shaped pit was then dug out and human bodies were laid to rest within the pit. One of the excavated mounds held four bodies. Soil was then piled up upon the bodies along with rocks and clam shells. Cremation ashes mixed with baked reddish clay were mixed in with the final layer of earth on top of the mound.
The location of the mounds, especially within this region, were generally along elevated surfaces. These mounds, in the picture above, are the end of a linear series of conical mounds that end at a promontory overlooking the Mississippi River (below). By placing the mounds at an elevated location they remained isolated from the flooding common to the shores of the Mississippi River.
All of the mounds within the park are located upon an elevated plain overlooking the Mississippi River, with tributary rivers, like the Yellow River adjacent to the entrance and Visitor's Center, carving deep valleys within the plateau. The plateau is comprised primarily dolostone (a more weathering resistant variety of limestone) and sandstone. Both provide a surface that is slow to erode away.
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| Prairie du Chien Group seen along the hike up to the mound plateau on the northern portion of the park. |
Geologically, the mounds sit upon the Lower Ordovician age (~480 million years ago) Oneota and Shakopee Formations, which make up the Prairie du Chien Group. These beds are primarily made up of dolomite, quartz sandstone, and sandy dolomite. There is also significant amounts of chert within the beds. Below the Prairie du Chien Group, you can also see the older, Upper Cambrian in age (~490 million years old), Jordan Sandstone along the parts of the park that border the Yellow River. Both of these formations were deposited when the area was inundated by a vast sea. The sea ebbed and flowed across the land, alternating periods of deep water deposits as well as shallow coastline deposits. When the water was shallower, sand would be deposited along the coastal waters. As the water deepened, limestones and dolostones (a variety of limestone that has a high concentration of magnesium within the crystal structure), were deposited from the bodies of sea life that slowly died over time.
Glacially, Effigy Mounds is in the Driftless Area of Iowa, meaning that it was never covered with glaciers. While these elevated plateaus did not feel the effects of glacial activity, the neighboring Mississippi River was carved out by glacial meltwaters, with glacial debris filling up the river valley up to 150 feet.
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| 3 Mounds |
While the park is primarily an archeological site, geology plays a major role. By providing the elevated location for the mounds, to suppling the materials used that were incorporated within the mounds, such as the dolostone, clay, along with the soil and sod, geology was a major contributor to the building of the Effigy Mounds.
References
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