Geology is Detective Work
Everyone likes a good book. Now days the genre has changed. With the proliferation of the internet, reading books may go the way of the Wooly Mammoth. Geologists, while having read their fair share of text books and technical articles also spend a great deal of their time (besides web time) reading the pages of the earth’s book. I once had a student who was always the last one up the hill. I just thought he was slow and out of shape. One day I saw him open his always bulging backpack and to my astonishment, a dozen heavy college textbooks fell out. After shaking my head, my only public comment was, “the only books you need in the field is the outcrop in front of you and your field notebook”. (He passed the class by an act of charity.)
In a previous Geologic Column, I wrote about how the present is the key to the past (and the future). Lets remember this rule as I discuss the turning of the pages of earth’s book.
Individual rock layers are commonly visible as you drive down the highway around Powder River County. (70 mile per hour geology!) They stick out visually in sharp contrast as they are usually harder than surrounding rock (which weathers away). They might be a cap rock protecting the softer sediment below or just a huge monolithic chunk of sand that sticks out of a hillside. My point is that each rock unit that can be easily identified as different from the surrounding rocks, has a different, unique series of events that led to it’s formation and preservation. This sequence of events is a snapshot on the page of the book I reference above.
The science of stratigraphy is the study of the origin of rock layers. (Biggest term of the week time!) Geologists use principles of “paleo-environmental stratigraphy” to determine the ancient environment that enabled the deposition of the sediment (any detritus such as dirt, sand, silt, clay up to boulders in size) that led to the hard rock we see today. Geologists use physical characteristics of the original sediment preserved in the rock to do the detective work which figures out the paleoenvironment (ancient conditions including climate, whether water had anything to do with the deposit, what happened shortly after the sediment was buried, etc.). Lets use a big ledge of hard sandstone over a soft layer of muddy shale as an example.
Where today is sandstone made? Locally, the Powder River moves a lot of sand that has been weathered off the land by that all too rare precipitation event. A walk down the river bottom will show you a lot of different environments for sediment to accumulate. The quite pools of water (a bowl) cut off from the main river flow, will always have a layer of fine mud covering the bottom of the sediment below (which is mostly sand/silt). Over time, the pool may fill up with mud. This lens of mud could then easily be covered by a thick blanket of sand from the next flood. So our sequence of a sand dish pool, filled with mud and covered with a blanket of sand is buried over the years by more such sequences. With the passage of time, the sequence hardens because of chemical changes from ground water and compression from the weight above. Presto-chango and you have rock and a rock layer sequence telling you a story. If nature does that long enough hundreds of feet of sand/shale/sand sequences accumulate and you have a rock formation form.
I collect all sort of fossil remains from the local rock formation named “Hell Creek”. It was formed more than 65 million years ago (Upper Cretaceous Period) by similar environmental conditions to that described above. (Have you ever seen an animal bone in a river bottom?) It is hundreds of feet thick, is very fossiliferous and the rivers that formed it were fed sand by the trillions of truckloads from the newly rising Rocky mountains to the west. If I take you to Hell Creek outcrops in the field, I can show you numerous sandy dishes filled with a fine muddy shale capped with a blanket sand. I can watch the process today live, real time (albeit slow motion) or I can see the results of the same process in the rocks almost anywhere in the county. All you have to do is look at the picture on the page. The present is the key to the past!
More on those fossils in a later Geologic Column.