A FALL OF GROUND OCCURRED AT THE INTERSECTION OF 1055 SUMP AND 1055 ACCESS WHEN A NOSE CORNER (RIB) FELL OUT, ALONG WITH SOME ROCK IN THE BACK. THE FALL DID NOT OCCUR ABOVE BELT ANCHORAGE, BUT NUMEROUS 7' SPLIT SET BOLTS DID PULL OUT OF A VERY SOFT GUAGE LAYER THAT THEY WERE ANCHORED IN AND WHICH WAS NOT VISIBLE UNTIL AFTER THE FALL. APPROXIMATELY 100 TONS OF ROCK FELL.