The Data in the Strata; Field Lessons in Sedimentology

To study sedimentology and the deposition of rocks at different stratigraphic layers, it is necessary to dig trenches or stratigraphic columns. When digging a trench, it is essential to be on a hill or butte, and it helps if it’s steep. Digging a trench is no easy task: building one consists of using a pick-axe to hack away at the rock surface, a spade to clean the hacked areas, and a shovel which is used to clear all the dirt away. A column should be precise, with even cuts, and a 90 degree vertical back wall, so that data is consistent throughout. Trenches should also be dug in steps which creates a set of stairs that show what is happening in multiple frames of time. We had to take care not to make a step on a contact between two different layers, so that the diagnostic features of the rock faces are available to analyze. It is important that you dig the trench yourself, as opposed to having other people dig it, so that you may learn as you dig. Seeing the process of trenching as it happens will help you understand the different rock layers as they are exposed.

 

When studying sediments, the layers of sediment can't  be used to measure time without careful examination. For example, while one sediment may take thousands of years to form a layer on the Earth's surface, but many only take a few years or even a day in some extreme occasions. This means that some points in time may be represented more than others in the sediment layers, and it shows that time isn't always represented in equal increments. Another complexity comes with erosion. Water can erode away old sediment layers and replace them with new ones, causing much newer sediments to be deposited just as far down as much older ones. The importance of knowing what amount of time sediment layers represent is showcased when paleontologists are collecting data on the age of a fossil because they have to know how old the sediments around the fossil are to determine the age of the fossil. The trenches are dug in order to find boundaries between rock layers which represent changes in the ecosystem throughout time.  Boundaries are made up of faces which consist of four types of stones: mudstones, siltstones, sandstones, or conglomerates. In addition, boundaries can have a mixture of the layers so it's not unusual to have a silty mudstone or a silty sandstone. Boundaries also represent a change in the ecosystem in a certain amount of time. For example, Dr. Bercovici explained that lakes can cause some layers to appear much older then they really are.  These changes can tell us what the ecosystem was like in that time frame. This gives us a better understanding of what life was like for the living organisms of that time and creating a time line for vertebrate paleontologists to contextualize their finds.