What our geology reveals about Mount Vernon

If you have ever walked in Mount Vernon, you will be familiar with the hill along which First Street (aka Main Street) runs. This is a narrow but fairly steep hill oriented in a NW–SE direction, and in geological terms, this hill is known as a paha.

Pahas are made of older glacial sediments capped by fine, wind-blown silt (loess) and were sculpted during the last glacial stage (~75,000–10,000 years ago) as the surrounding area was eroded and flattened. Other similar hills stretch out in the same orientation across several counties in eastern Iowa.

Underneath the layers of soil and sediment lies the bedrock of limestone and dolostone that you see at Nature Park (an old quarry) and Palisades-Kepler State Park. These rocks are much older (about 420–445 million years old!) and contain marine fossils such as crinoids and brachiopods. Limestone is formed in shallow, tropical ocean environments, which tells us that Mount Vernon was once closer to the equator and covered in warm, shallow seas similar to those near southern Florida or the Bahamas.

This central part of the continent spent hundreds of millions of years covered by shallow seas, resulting in bedrock that largely consists of sedimentary rocks like limestones and dolostones.

You might also have noticed large boulders at the edges of farm fields or even cobbles and pebbles along the rivers that look entirely different. These are often more colorful rocks with sparkly minerals of different sizes and shapes—different types of igneous and metamorphic rock. These rocks were transported south from Minnesota, Wisconsin, and the Upper Peninsula of Michigan by glaciers and rivers draining glaciers, and they provide a very different origin story from the local rocks. For the most part, these rocks are even older (more than one billion years old!). Some originated as lavas when the continent began to pull apart along a fault reaching from the Lake Superior region down through Iowa and into Kansas. Other rocks are metamorphic rocks that originated in huge mountain belts (like the Alps) that used to exist in what is now Minnesota, as the continent was just being pieced together.

Each rock has a unique history and can provide a valuable glimpse into the past.