Had a highwall failure on the surface area, between #3 & #4 drift mouth entries. The incident occurred from repeated freezing/thawing of the high wall. The rock measured approx. 30' tall X approx. 2' to 20' wide at its widest point X inches to 8' thick at its deepest point. All employees were withdrawn safely. The incident was reported to the 24 Hr. Reporting Hotline.