 The type of field stones left through glaciation, are known as glacial erratics.In Canada and the northern United States, the advance of the Laurentide Ice Sheet pulverized bedrock, and its retreat deposited several dozen meters of unsorted till in previously glaciated areas as far south as New England and the Upper Midwest.Although a coarse layer of glacial ablation would settle on top of the deeper lodgment till, it was these more deeply set stones that would prove a persistent challenge for settled human agriculture because they would be frost-churned into surface soils during harsh winters.