Geology: What's make a group of bedrock a formation?
Basically a formation is just a layer or group of layers of rock that is both distinct enough to be identifiable in the field and large enough to show up on a geological map. Usually it is many nearly identical layers grouped together or just one large layer. Its not a hugely precise definition but it is a lot like the term "species" in biology, it's a working definition, created more for usability than precision.