Dick said:
"In New England, we get a phenomenon called "frost heaves". It
happens when the ground freezes, melts, and refreezes in late
winter....."
This phenomenon is present in Minnesota and the Dakotas as well.
I suspect that Montana and parts of Wisconsin and Michigan also
see it. If you live above 45 degrees North latitude and not on
the coast I would expect that one must go down 48 inches or more
to assure being below the "frostline". As the latitude increases
the depth increases. When the outside temperature is below
freezing (day and night) for several months at a time and the
ground has any moisture at all, things will freeze down to 40
inches.
Tod Olson, K0TO
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