Do you think maybe with all that rock in the soil, the rock was acting like a solar storage unit and keeping the soil a few degrees warmer and so it never froze?

Could be. In fact, that's probably exactly what happened. Thanks Larry! I never thought of that, but you must be right.
(When I read your comment title "rocks in the soil" I expected it would be followed somewhere by the phrase "rocks in the head." *HAR*)