It’s a little easier to understand rocks when you can relate them to your surroundings. We’ll take a look at what is far below the ground in Indiana, and then examine what you might find at the surface. (Remember the tectonic setting that creates sedimentary rocks?) Looking at these rocks can give you clues you into the ancient history of Indiana and help you to understand why one of Indiana’s greatest resource is limestone, a sedimentary rock.
What’s at the Surface?
Click on the map for a larger image.Geologically, Indiana is underlain by three types of material. Starting at the bottom, igneous and metamorphic rocks form the continental crust under Indiana. This is covered by a few thousand feet of early Paleozoic sedimentary rocks. In southwestern Indiana, the sedimentary rocks are much thicker and include younger Paleozoic rocks at the surface. In central and northern Indiana, a thick layer of glacial deposits covers the early Paleozoic rocks, much like icing on a cake.
The physiographic map to the left gives you an indication of where Indiana is buried under glacial sediments. The green and blue colors represent the mostly glacial deposits that have buried all the rocks. The orange and reddish colors represent relatively thin layers of sediment that cover rocks exposed at or near the surface (Indiana Geological Survey).
Northern and Central Indiana
In central and northern Indiana, the glaciers eroded away the rocks at the surface and buried them in glacial sediment that ranges from 50 to 500 feet thick. But all is not lost. In this glacial sediment, you can find rounded boulders and rocks the glaciers dragged down from Canada. Most of these boulders are igneous and metamorphic rocks native to central Canada. Because of the glaciers could have dragged anything down from Canada, you have to look at each individual rock to figure out its variety.
This rock fence in Southport, IN is built out of small igneous
and metamorphic glacial boulders that can easily be found in yards and
fields. (C. Thomas)In Central Indiana, many people use these glacial boulders as decorative stones. They sit at entrances to driveways, they sit landscaping, and in older houses they are used in fences. We’ll learn more about how glaciers can drag rocks hundreds of miles in a later part of this course.
This pile of rocks along Emerson Ave. in Indianapolis are also
glacial boulders—likely piled here after nearby construction of
office buildings. As the sign shows you—these are popular for landscaping.
(C. Thomas)