Four Mile Dam Formation
The Four Mile Dam Formation, also called the Four Mile Dam Limestone, is a Formation (geology), geologic formation in Michigan. It preserves fossils dating back to the middle Devonian Period (geology), period. Fossil content Vertebrates Acanthodians Conodonts Placoderms Invertebrates Brachiopods Corals Gastropods Sponges Trilobites See also * List of fossiliferous stratigraphic units in Michigan References

* Devonian Michigan Devonian southern paleotemperate deposits {{Michigan-geologic-formation-stub ...
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Geological Formation
A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exposed in a geographical region (the stratigraphic column). It is the fundamental unit of lithostratigraphy, the study of strata or rock layers. A formation must be large enough that it can be mapped at the surface or traced in the subsurface. Formations are otherwise not defined by the thickness of their rock strata, which can vary widely. They are usually, but not universally, tabular in form. They may consist of a single lithology (rock type), or of alternating beds of two or more lithologies, or even a heterogeneous mixture of lithologies, so long as this distinguishes them from adjacent bodies of rock. The concept of a geologic formation goes back to the beginnings of modern scientific geology. The term was used by Abraham Gottlob Wer ...
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