DinoByte Wednesday: Rock Formations

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.

The Grand Canyon has many formations exposed (top) that differ in rock composition and appearance. These formations illustrate uplift and tilting events, as well as more than two billion years of rock deposition (bottom).

The Grand Canyon has many formations exposed (top) that differ in rock composition and appearance. These formations illustrate uplift and tilting events, as well as more than two billion years of rock deposition (bottom).

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).

An artistic rendition of a paleontologist as she uses exposed rock to infer past environments.

An artistic rendition of a paleontologist as she uses exposed rock to infer past environments.

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.

DinoByte Wednesday: Geologic Time

Welcome to the first DinoByte Wednesday! In preparation for the upcoming 2014 DIG Field School, we’ll be publishing a series of blog posts on our DIG team, research, and fieldwork in the Hell Creek area of northeastern Montana. Today, we’ll discuss the geologic time scale as it relates to our field site.

The Earth is 4.6 BILLION years old, and the geologic time scale breaks this long amount of time into smaller units. These units, arranged from longest to shortest, are eons, eras, periods, epochs, and stages. The divisions between units are based upon major geological and paleontological events. At our field site in Hell Creek, Montana, the fossils indicate dinosaurs lived there during the Phanerozoic Eon, of the Mesozoic Era, during the Maastrichtian Stage of the Cretaceous Period.

Geologic Time Scale

Geologic Time Scale, modified from the Geological Society of America

The Mesozoic Era is the “Age of Dinosaurs,” and the dinosaur fossils we find in the field are 68 million to 66 million years old! We will find dinosaurs during the DIG Field School (as well as turtles, crocodiles, fish, mammals), but one of the things that makes our field site so special is that in this area there are rocks that preserve the last two million years that non-avian dinosaurs inhabited the earth, the layer that shows dinosaur extinction, AND the first one million years of the Paleogene Period.

The Paleogene Period (and the Paleocene Epoch) marks the start of the Cenozoic Era, the “Age of Mammals.” At our field site, in addition to Cretaceous dinosaurs, we find fossils of the mammals and other animals that survived the Cretaceous-Paleogene Mass Extinction that killed the dinosaurs (except birds). That makes this area one of the best places in the world to study dinosaur extinction AND the subsequent recovery of mammals.

Cretaceous-Paleogene rocks in the Hell Creek area of northeastern Montana

Cretaceous-Paleogene rocks in the Hell Creek area of northeastern Montana.

Most of the geologic layers in Montana that contain dinosaurs lay at the surface. Next week, we’ll DIG further into rock formations and uncover why these fossils sit at the surface when they are up to 60+ million years old.

Strata Column by artist Ray Troll, http://www.earth-time.org/trollart.html

Strata Column by artist Ray Troll

P.S. Are you wondering how we can determine the age of the fossils found in the Hell Creek area? These are much too old to use carbon dating, so we can use nearby rocks as clues, particularly layers of ash and coal. Stay tuned for more on dating fossils in a future DinoByte post!

(Most of this information came from the excellent book Dinosaurs Under the Big Sky by Jack Horner)