Scientists surmise that these scour marks on the landscape of the north american plains were caused by giant icebergs being blown across the windswept surface of the ancient lake Agassiz. Being gigantic, these icebergs dragged along the soft soils of the bottom of the lake, leaving their marks to be seen even today. The marks are often several miles long and many feet deep.

Lake Agassiz was formed 7,000 to 10,000 years ago as a result of the melting of the continental ice sheets at the end of the last ice age. Check out the size of this monstrous lake on the map below. It makes the great lakes look like ponds.