Lots of slate there
Actually, there's no slate at all there. It's shale, sandstone and conglomerates in alternating sequences (due to coast line advances and retreats). Slate is a metamorphosed shale. Being far from the ancient tectonic collision zones, the rocks in the Catskills are almost entirely sedimentary. Metamorphism occurred further east in New England, also New York City. The sedimentary beds of the Catskills are the erosional remnants of mountains that were raised in the collisions (~400-350 million years ago).