December 2013

Rocks, Geology, and Coal Mining

Contributed by Cindy Blobaum, author of "Geology Rocks!" and Naturalist, Dallas County Conservation

The story of Iowa started long before people were around to witness and record what was going on. To uncover the story of how Iowa was formed and has changed, geologists look for clues in rocks, both present and missing. While most people don’t consider Iowa very rocky, farmers removing stones from their fields might argue otherwise!

Excluding field stones, much of the geologic history lies hidden under our fertile soil, making the best place to look for rock layers along our streams and rivers. From what geologists can see, or guess from what is missing, they believe that earthquakes, volcanoes, floods and ice have all had a role. Through long periods of time, the land has formed, eroded, and reformed through these slow but never-ending processes. And while there are many similarities in the geology across the state, each region has unique qualities that have played a part in where people have settled and how they have used the land.

**Cindy found this book in April 2016 and highly recommends! Wisconsin, Illinois, and Iowa Rocks and Minerals by Dan and Bob Lynch.

Marion County

Currently Iowa is far from an ocean or vast lake, but it hasn’t always been that way. The rocks show that Iowa has been under water, either liquid or frozen, for much of its history. Ancient seas covered Iowa while it was located near the equator. While underwater, the land became covered with layers of sediments, including many aquatic animals that became fossilized. Pressure from upper layers and minerals in the water worked together to cement the sediments together, forming rocks including limestone, dolomite sandstone, and shale.

View the "Rocks and Fossils" album in Google Photos.

At the same time, the continental plate on which Iowa sits wandered around the globe, taking a trip that has lasted millions of years. At one time the plate split apart, creating a rift that filled with magma. Then it moved back together. Several other times, it crashed into other plates. These crashes caused parts of the land to get pushed up out of the water.

At one point in time, the land was higher in Wisconsin. A great river drained the land and soil into Iowa, covering much of the northeastern part with a silty delta. The climate was hot and humid. A large area that includes what is now Marion County was part of a warm swamp. Giant club mosses, horsetails, and coniferous trees created a jungle-like environment. As the plants died, they became covered by water and sediments. Instead of rotting away, as they were compressed they became coal.

Through more plate movements, the land underwent several long cycles of rising and drying out, then sinking and being covered with water.

Even after the most recent land rise, the land became covered with water. Frozen water, in the form of mile-thick glaciers, crept across parts of Iowa at least four different times. The glaciers pushed a mix of loose gravel and soil, called till, across Iowa. This glacial till forms the base of Iowa’s rich farming soils.

When the climate warmed, the glaciers melted. Enormous amounts of water and ground up rocks rushed across the land, cutting new river valleys. After the water moved on, the sediments dried. Strong winds picked up the sediments and carried them across the land. Much of Iowa is covered by this windblown sediment, called loess.

Mining Iowa’s Treasures

Coal was mined in southeastern Iowa as early as the 1840s by early settlers. Most of it was burned to heat homes and businesses. When the railroads arrived, a lot of coal was needed for the steam powered locomotives. Everyone from families to companies across southern Iowa dug for coal and registered mines. Some coal seams were close to the surface, while others required digging shafts up to 245 feet deep. The coal layers that were worth mining could be anywhere from around 14 inches thick to over 42 inches (nearly 4 feet) thick. Within 20 years, rail tracks crossed the entire state and Iowa mined more coal than any other state west of the Mississippi River. Between 1860 and 1967, over 400 mines were registered in Marion County alone.

While many of us do not remember active coal mining in southern Iowa, the Marion County Development Commission produced a wonderful video in 1997 documenting the coal mining history. Check the list of oral history participants at the end of the video. None of these folks are still with us today -- are any of them your relatives?

Small towns sprung up around the larger coal mining operations. Coal production in Iowa peaked at 9.3 million tons in 1918. However, by the 1920s more people were using cleaner burning coal that was mined in Illinois. The coal mines of Iowa gradually shut down, although some were active until the early 1970s. With the closing of the mines, many of the towns also disappeared.

The main street of Buxton, a thriving community in 1915, is lined with homes of coal miners, including many African-American families.

Ponies and small mules were used to pull carts loaded with coal through the low-roofed mines.

In much of the county, the removal of coal created problems for future use of the land. The rocks directly above the coal created acid run-off when it was exposed to air and rain. Plants could not grow. Work has been done to cover both the strip mines and the spoils piles with deep layers of soil. These piles are planted with certain crops to return the land to productivity.

Make your own coal!

It’s hard to believe, looking at a hard, shiny, black lump of coal, that it’s a nugget of stored plant energy!

Here’s a way to see the beginnings of how coal is made deep inside the earth. With a grown-up’s help, cut a 2-liter soda bottle just below the shoulder. Fill the bottle with plant materials like leaves and grass clippings. Cover with water. Press a plastic lid, rim side up, down on the plants and weight in with rocks. Use the cut-off top to make a seal that will help keep out the air. Put the bottle in a warm place and push down on the sliding seal every day. Watch as your plant parts get compacted together.

As prehistoric plants fell to the ground, sometimes they were buried by layers of other plants. Without air, they didn’t decompose. As more and more plants fell down, the ones on the bottom were heating up and getting compressed. Over millions of years, as the water was squeezed out, this plant material became coal!

Used with permission, page 83, Geology Rocks! Available at