Roof fall was discovered at intersection of the inby recovery chute with the inby ladder located near 3 block of 22D. The fall was approx. 20' long, 16' wide, and 8-9' high. Possibly due to unforeseeable small area of unconsolidated shale with slickened sides just above the anchorage. No injuries. No problems with training, PPE, mining machines or compliance issues.