More recently, during the Pleistocene Epoch, the sandstone bedrock was carved into today’s familiar canyons and formations by the eroding action of glacial meltwaters. The Punch Bowl on Trail 3 is an example of a pothole that was scoured out by glacial erratics caught in swirling backwash. Erratics are pieces of bedrock from Canada that were carried here by the glaciers. Some of the larger erratics, or boulders, can still be seen in Boulder Canyon on Trail 9, and smaller ones, pebbles, in Sugar Creek.
In the last few hundred years, the wind and water erosion of the sandstone has continued at a slower pace. The gorges are very similar to what was seen by Native Americans and pioneers.