Mawson A is directly underlain by locally intensely folded sediments of the Beacon Supergroup. The intensity of folding decreases downwards from the erosion surface over a few tens of metres. Within the zone of most intense folding, metre and decametre scale isoclinal folds have formed during complex brittle failure of interbedded sandstone and shale while cataclastic flow occurred in some coal seams. Strain is partitioned into a zone subparallel with the Mawson Erosion Surface.

The style and distribution of deformation is closely analogous to that observed locally beneath the Sirius Formation, a known glacial deposit. This suggests that both the sub-Mawson and sub-Sirius strain resulted from similar processes. We believe that the deformation resulted from ice contact drag folding at the base of a dry-based glacier.