grasping the rock with both hands , inhaling sharply as it loosened and unexpectedly threw her back a step with its weight , she swung it against the insecure beam with all the strength she could muster vibrating , the wood cracked down the middle , and then the overhead beam slid downwards a slight but discernible shift , enough to disturb the structure and release a shower of rubble and dirt over her head ; and the same happened as she struck the beam another time , only the dirt continued to spill after the rubble stopped .