Back in the Ordovician time period, Indiana was covered by a shallow sea and located in a tropical climate—much like the Bahamas of today. (Image from The Paleontology Portal)
As you learned in G135, the idea of “geologic time” was coined by early geologists to explain the vast amount of time represented by Earth's history. Geologic time is no different from the time shown on your watch or cell phone: it’s the vastness of geologic time that is difficult for humans to grasp.
In this laboratory, you will explore the geologic history of Indiana in the context of geologic time. You will also look at the tools that geologists use to interpret the timing of past events, and how they correlate events in different places.
This overview is followed by directions for completing your workbook exercises. Please complete this overview prior to beginning your lab exercise.
BEFORE continuing this overview, please read the following pages in your lab book:
Pages 39-49: Please read all the text on these pages for a review of geologic time. We will cover many of these concepts here in the module content, but this is a good pre-reading to get your mind thinking about geologic time and geologic history.
Page 59-61: Read the introduction to this part of the lab to learn how geologists correlate rocks over large distances. We will not cover this in-depth in this module content, as the material is covered clearly in the lab book.
The Geologic Time Scale
Geologists often use metaphors to explain how massive the geologic time scale is. Let’s use a metaphor to give you a sense of the size of geologic time - if you were to count to 4.6 billion, saying one number every second, how long would it take you? It would take you 145 years to count to a number that large!!Pages 44 and 255 of your text have geologic time scales for Earth's history. This scale is somewhat skewed: the table doesn’t visually show the size of each time period. The scale in the text exaggerates the past 550 million years because most Earth events of interest (to us) have only happened in this time frame. The diagram on page 44 is much more simplified in that it only shows eras and periods, and does not show eons or epochs or any further subdivisions. The top of the time scale represents today, the bottom represents the beginning of Earth history about 4.6 billion years ago. The numbers represent years before present - on both diagrams the unit on these numbers is millions of years before present. Take a few minutes to orient yourself with the geologic time scale and the vast amount of time it represents.
How was the Geologic Time Scale Created?
The time scale was created in the 1800s and early 1900s and continues to be refined today. The scale’s "time periods" are an arbitrary way for scientists to easily communicate about events in Earth’s history. Made in the 1800s and 1900s, the main time period divisions are based on the careful observations of rock exposures all over. Before the mid-1900s, no absolute ages were known, only relative ages. The advent of radioactive dating allowed for the establishment of absolute ages for the various time periods. Divisions between time periods are based on extinctions seen in the rock record, major plate tectonic events, and evolution of certain key fossils, or even climatic changes
Estimates of geologic time were initially based on observations by early geologists that noted Earth processes happen extremely slowly, the Earth's rock record could be correlated across different countries and continents, and that fossils could provide a general sequence to the age of materials. Advances in chemistry and physics allowed geologists to use radiometric dating, or to measure small amounts of radioactive material present in all Earth material. Using radiometric dating, geologists were able to determine the approximate ages when certain types of Earth material formed. To geologists surprise, their "very rough" estimates and sequence of events predicted by simply looking at rocks were shown to be correct.
Geologic Time and Indiana
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.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.
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.
We will look at specific evidence of these time periods in our field trips, especially those to Turkey Run and McCormicks Creek State Parks. If you have extra time, go visit Falls of the Ohio State Park for an interesting look at the Devonian time in Indana. The Indiana State Museum also has a wonderful display of Indiana's geologic past!