Then, either by hydraulic wedging or frost-cracking the block can be forced out.
2.
The freezing and thawing action of the ice enlarges, widens, or causes further cracks in the bedrock as it changes volume across the ice / water phase transition ( a form of hydraulic wedging ), gradually loosening the rock between the joints.
3.
In such cases, the loosening and detachment of blocks appears to result from a combination of ( 1 ) chemical and physical weathering along joints, ( 2 ) hydraulic wedging driven by smaller rock fragments getting into existing cracks, ( 3 ) crack propagation from stresses caused by impacts of large clasts already in transport by the river, and possibly ( 4 ) crack propagation driven by flexing resulting from pressure variation in the overlying water during floods.
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