What is a geologic formation? We can think of a formation as a unit of rock. Each unit is a package of sediments, such as sand, silt, and volcanic ash, that cover an area large enough to be mapped, and contain a particular group of fossils. As time passes, different sediments are packed on top of one another forming different geologic layers of rock. So, the top layer of rock should be the most recent formation and the deeper we dig down vertically, the older the rocks and corresponding fossils, right?
It’s a bit more complicated than that, as sometimes rocks exposed at the surface are older (or younger) than you would expect. Changes in the earth’s landscape are represented in the geology of the different rock layers. Then, movement of deep molten rock pushing upward to form mountains can rearrange different rock layers and can tip originally flat layers. Similarly, changes in the movement of different bodies of water like streams, lakes, and oceans cut through the rock layers, erasing younger sediments on the surface, and exposing older sediments in stream and road cuts. The Grand Canyon is a picturesque example of geologic change, with billions of years of exposed rock, tilted layers, and river channels cutting through layers.
Having trouble visualizing how this works? Watch this short video of paleontologist Kirk Johnson as he explains how rocks can change over time, using pancakes!
In Montana, many of the formations are from the Mesozoic Era, but the site of the DIG Field School contains formations from both the Mesozoic and Cenozoic Eras (remember last week’s post?). By looking at the different rock formations we are actually trying to piece together clues about how the environments differed during each of these geologic time periods. We use the rock formations to guide our interpretation of the landscape and environments experienced by the dinosaurs, mammals, crocodiles, turtles, amphibians, and fish when they were alive here, 68-65 mya (million years ago).
So what packages of rocks do we encounter during the DIG Field School? Next week we’ll discuss the specific formations found at our field site, namely the Hell Creek and Tullock formations.