structure that was deposited in the northeastern United States during the Middle and Late Devonian Period (the Devonian Period began about 416 million years ago and lasted about 57 million years); it is named for exposures studied in the Catskill Mountains of New York. During Middle and Late Devonian time, numerous streams flowing from the Appalachian Highlands to the east constructed a large compound delta along the northeastern regions of the United States. Late in the Devonian Period, the shoreline was pushed back to western New York and northwestern Pennsylvania. A broad coastal lowland, extending more than 320 km (200 miles) across, was formed; at the same time, the submerged portion of the delta was growing and extended into Ohio.