Orohippus, Index Fossils and the Tertiary.
The evolution of certain animals can 'date' various strata and a good example is the lineage of horses. starting with the earliest forms such as Orohippus up through the modern horse their evolution bridges tens of millions of years. The Chart below gives the statigraphic picture just east of us from the tressetrial record of the fossil horses. This is the same time period we are dealing with in the marine environment which is forming the Willapa Hills from the deposition, uplift and erosion cycle.
