The original post: /r/askscience by /u/RW_archaeology on 2024-10-01 15:00:13.

There is a major part of geology I don’t think I understand. I often see geological maps of areas like this one showing different bedrock by periods. I live in an area of Indiana where most of the region’s bedrock dates to the Pennsylvanian period. I understand how this date was established with index fossils and dating methods, however, the entire map goes against what I understand of geology. I was under the impression areas had bedrock sorted horizontally in stratigraphic layers, with different layers dating to different periods as this image shows. Why then do geological maps show a single period dominating an area? Are geological maps just showing the top layer? If so does that mean I could dig down where i live in Indiana and eventually hit earlier-period rocks with earlier-period fossils? With how many valleys and quarries there are in my area you’d think there’d be an outcropping from one of these earlier periods, but that doesn’t seem to be the case, hence my confusion. Thank you for any responses