The base of the region was formed 180 million years ago. At that time a lake was formed that allowed sediment to slowly compress into sandstone. The Warrumbungles are the remnants of a large heavily-eroded shield volcano which was active from 13 to 17 million years ago. The volcano is estimated to have been high and wide. It formed as volcanic explosions occurred over millions of years. The remaining complex rocky formations are what is left after millions of years of erosion.