Bluenose Lake thrust moraines

This is a very interesting paper which illustrates graphically the effects of thrusting in situations of compressive flow.  the authors describe an area where Wisconsin glacial ice is still present under a thick cap of till neat the Arctic coast of NWT.  They describe a situation in which the ice has butted up against a transverse hill mass, leading to compressive flow and shearing.  In the diagram above, the lower layers of ice are relatively inactive, and they are being overridden by more active ice layers moving along shear planes.  Large amounts of debris are transported along these planes by thrusting, and strips of moraine are visible at the ice surface where these planes emerge from the glacier.  When the ice has melted away, these ridges are up to 100m high -- very dramatic landscape features.

It follows from this work that the erosion of the bedrock is going on on the upslope section of the hill mass against which the ice is pressing.  If we transpose this situation to the Preseli Hills area, the erosion zone would be equivalent to the north of the uplands, not the south. 

Source:
Géographie physique et Quaternaire, 1999, vol. 53, n° 2, p. 287-295, 9 figs.
LA MORAINE DU LAC BLUENOSE (TERRITOIRES DU NORD-OUEST), UNE MORAINE À NOYAU DE GLACE DE GLACIER
by Denis A. ST-ONGE et Isabelle McMARTIN