The Devonian is sometimes called "The Age of Fish" since they were especially abundant and diverse at that time (more diverse than they are today with two extra taxonomic classes which no longer exist). Fish were the most advanced vertebrates at the time, amphibians had yet to evolve (but would do so by the Late Devonian a few tens of millions of years away).
Dunkleosteus - A Devonian Placoderm (armored fish) which reached 10 m in length
Life had come out onto land. Starting with primitive non-vascular plants and arthropods a few tens of millions of years earlier, by the Middle Devonian there were entire forests of trees that would appear alien-looking to us today. One important locale preserving evidence of these forests is right here in eastern New York - the Gilboa fossil forest site in the Catskill Mountains. These forests were crawling with insects and arachnids, but no amphibians, reptiles, mammals, or birds yet existed to prey upon them.
Reconstruction of the 380 million-year-old Gilboa forest
These forests formed on a enormous river-floodplain-deltaic system called the Catskill Delta by geologists today (think of the Ganges–Brahmaputra Delta in our modern world). The reason the above reconstruction looks like a subtropical swamp is because New York was subtropical at the time because, throughout the Devonian, proto-North America (called Laurentia by geologists) was in the Southern Hemisphere but drifting northward towards the equator.
Paleogeographic map modified from Ron Blakey, NAU Geology
So here's a thumbnail sketch of what led to the Catskill Delta system. It's WAY oversimplified but that's OK, this isn't a geology class. Just keep in mind that the details are much more messy and complex that the simply picture I'll outline here.
At the start of the Devonian Period (416 million years ago), the Hudson Valley was covered with a shallow, subtropical body of water called the Helderberg Sea. How do we know this? Because we can stop on the side of the road a mile from where I'm sitting and typing this and examine a whole sequence of limestone chock full of marine invertebrate fossils.
Brachiopods in Early Devonian New Scotland Formation, Kingston, NY
Throughout the Early Devonian Period, a microcontinent called Avalonia was approaching us (due to plate tectonics as blocks of crust are always moving around the surface of the Earth). Volcanic activity associated with this approach even left layers of altered (because it fell through seawater) volcanic ash in this sequence of limestones allowing us to radiometrically date them.
As Avalonia approached and collided, it resulted in mountain building. Just as India colliding with Asia in the past few tens of millions of years resulted in the massive Himalayan Mountains, Avalonia colliding with Laurentia (proto-North America) resulted in a massive Himalayan-scale mountain range as well. They were called the Acadian Mountains and were over in what's now New England.
Acadian Mountains and Appalachian Basin (Frances Fawcett, Illustrator)
As the Acadian Mountains grew, muddy sediment started coming down into the nice clear sea that existed here in the Hudson Valley (Appalachian Basin). The bottom sediments changed from carbonate (limestone) to clay mud (shale). Fossils are different in these rocks - there are still marine invertebrates but different species better adapted to the changes in water conditions. Many delicate filter-feeding organisms disappeared.
Marine worm feeding trace in the Early Devonian Esopus Shale, Kingston, NY
As the Acadian Mountains rose, pulses of sediments came eroding down into the shallow sea and sea levels also fluctuated over time (we're talking millions of years of time). This resulted in different sequences of rocks being deposited into the foreland of the Acadian mountain belt. Eventually, the sea filled up (first closest to the mountains and then further away) and became a low-lying swampy river/floodplain system - the Catskill Delta.
Now we've set the stage. It's about 385 million years ago (the early part of the Middle Devonian Period). Proto-North America (Laurentia) was rotated and straddling the equator which placed our area in the Southern Hemisphere subtropics. The microcontinent of Avalonia was colliding pushing up the Himalayan-scale Acadian Mountains. The Catskill Delta system was starting to form but a shallow sea still covered this area (and extended far beyond just the Hudson Valley covering much of what's now eastern North America. The seafloor was muddy.
Marcellus Shale (from Climate Change 101)
A widespread distinctive black shale formed from these seafloor muds which today is known as the Marcellus Formation. More on this in my next post.