After the Grenville Mountains started to erode in New York State, the important geologic event that occurred was the formation of the Catskill Delta. The Catskill Delta is a geological feature formed by sediment deposits in an ancient delta environment during the Devonian period. This event is significant as it contributed to the shaping of the landscape in New York State and played a role in the region's geologic history.
Therefore, the correct answer to the question is:
3. The Catskill Delta formed.
