Geology: What's make a group of bed 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.
According to Wikipedia, and perfectly consistent with basic historical geological textbooks, a formation is:
...the fundamental unit of lithostratigraphy. A formation consists of a certain number of rock strata that have a comparable lithology, facies or other similar properties. ... The concept of formally defined layers or strata is central to the geologic discipline of stratigraphy.
Typically, formations are traceable across some distance.