Geologic Time and Indiana
To localize our discussion of geologic time, let’s take a look at geologic events that occurred in Indiana (this brief history is biased towards central Indiana and ignores some events).
First, Indiana did not exist prior to 1.5 to 1.4 billion years ago. Somewhere in this time frame the early North American continent expanded in size to cover the present-day Indiana. You can still find these rocks, but you’ll have to dig down thousands of feet. Drill cores of these rocks are on display at the Indiana State Museum. Then in the Cambrian time period, Indiana became covered with 100s of feet of sand and mud eroded off of a distant Canadian shield, volcanoes near present day Arkansas, or the uprising Appalachian Mountains. These rocks are again buried, but only a few hundred feet below the surface in some areas.
Did Dinosaurs Roam Indiana? Nobody knows! Dinosaurs are found in rocks of Mesozoic age, but no Mesozoic rocks are found in Indiana (or any nearby state). One exception is central North Carolina: there, small valleys (called rifts) opened up wide enough and deep enough to collect Mesozoic sediments, and in these sediments, dinosaurs were found. These deep valleys were spared from the erosion that wiped out the rest of the Mesozoic rocks in the Eastern U.S.
If you can, take a 10-15 minute break here. The next section is ten pages, and you’ll need your brain to reset to help you absorb all this information! But, don’t worry; there are many videos and animations to watch in these upcoming pages.
Between the Ordovician and the Carboniferous time periods, a majority of Indiana was submerged under a shallow warm ocean. Great amounts of fossil-rich limestone were deposited, intermixed with sand and mud eroding off of the far away Appalachian Mountains. All of these rocks are exposed in various places around southern Indiana. Then in the Mississippian time period, Indiana emerged from these seas, only to be covered by massive amounts of sand and mud, which were themselves covered in the newly evolved plants and trees. (These decayed plants later formed the thick coal beds.) While most of these rocks have been eroded off Indiana, they still remain exposed in southwestern Indiana. (The link in this paragraph is from the Children’s Museum of Indianapolis. It is not required reading.)
Then nothing. Between the Permian and mid Tertiary time periods, any evidence of what happened in Indiana has long been eroded away. All the evidence was washed into the ocean. That’s over a 100 million years of time missing from Indiana! The next significant event recorded in Indiana was the advent of glaciers within the last few million years. Northern Indiana records the scarring and massive deposits of sediment that these glaciers left behind. The glacial debris contains numerous fossils of ice age creatures. Southern Indiana was spared by the glaciers, which is why we can easily find the rocks deposited in the Paleozoic. In northern Indiana, the Paleozoic rocks are buried under 10s to 100s of feet of glacial sediment. We’ll talk more about this in the upcoming Modules.
The hills of Brown County State Park, Indiana expose Mississippian rocks formed as Indiana transitioned from a shallow sea to dry land (Indiana DNR).
Now that we have climbed the mountain of geologic time, we are ready to move on two the next major pillar of geology: plate tectonics.