The majority of bedrock in the Catskill Mountains was formed during the Devonian Period, which lasted from about 419 to 359 million years ago. This period is part of the Paleozoic Era.

Here's why:

* The Catskill Formation: The Catskill Mountains are primarily composed of the Catskill Formation, a thick sequence of sedimentary rocks. These rocks were deposited in a vast delta system that formed as the Appalachian Mountains rose to the east.

* Devonian Delta: The Devonian Period is when this delta system was most active, leading to the accumulation of the vast amounts of sandstone, shale, and conglomerate that make up the Catskills.

* Erosion and Uplift: After the Devonian, the Catskills continued to be shaped by erosion and uplift, but the foundation of the mountains was laid down during this period.

While there are some older rocks in the Catskills, the Devonian Period represents the most significant era in terms of the bedrock that makes up the mountain range.