If a scrap of dry land was to be found around New York, it would've looked and felt like Hilton Head Island, climate-wise. But dry land was in scarce supply.
That's because a large section of what are now the states of Pennsylvania, New York, West Virginia, Maryland and Ohio lay submerged beneath a sub-tropical sea.
In fact, water covered most of the world. Fish were the "boss" life-form of the planet. Their only competition was primitive plant life on land.
This shallow ancient sea—geologists refer to it as the Appalachian Seaway—stretched from the Gulf of Mexico to upstate New York.
Keeping company with the fish in the briny depths were microscopic plankton. And as these plankton died, a massive layer of dead organic material—think of it as green slime or "organic road kill"—collected on the seabed.
For hundreds of millions of years this gooey substance piled up.
Then an epochal tectonic event occurred.