Ancient Glacier Pot Holes Potholes were ground out of granite during high water of the Glacial Age by whirlpool action of water and gyrating stones of varied sizes. Some of the grinding millstones may still be seen in the smaller potholes. Over 50 such potholes exist in this confined area known in Indian Days as “Salmon Falls”. These potholes vary in size from 6 inches to 39 feet in diameter, the latter being the world’s largest pothole of record. See the MOVIE HERE. Learn more about this site HERE.