Formation
Also: geologic formation, rock formation
A distinct, mappable body of rock laid down in a particular interval, used to organize the fossil record. South Carolina names like the Ashley and Chandler Bridge formations place a fossil in time.
A formation is the basic unit geologists use to organize layered rock: a distinct, mappable body of sediment laid down over a particular span. Naming the formation a fossil came from is one of the most useful facts about it, because it ties the specimen to a known age and environment.
South Carolina's fossil-bearing formations
The Lowcountry's classic units include the Ashley Formation (early Oligocene, roughly 27 to 29 million years ago) and the overlying Chandler Bridge Formation (late Oligocene, about 23.5 to 24.7 million years ago), both famous for shark teeth and marine mammals, plus younger Miocene deposits. Knowing the formation pins down the geologic age and helps confirm which species should be present.
Why it matters for collectors
Formation is a core part of provenance and supports honest identification — a tooth's patina and matrix should match the formation it is said to come from. We describe locality in general terms to protect dig sites, a practice our guide to South Carolina's fossil formations explains in depth.