Why are the North Face of the Ben and Carn Mor Dearg so different? 

The walk in to the North Face is one of the iconic Scottish winter approaches, taking you into some very impressive rock scenery in the heart of the Ben Nevis igneous complex. I have been asked why there is a difference between the Ben Nevis side of the valley and the Carn Mor Dearg side - and the assymetry is very striking. The reason is two fold -
1) The North Face spends a lot more time in shade than the South-West face of the Carn Dearg ridge. That means glaciers stayed there long after the glacial maximum allowing them to eat into the high.
2) The geology is different. Ben Nevis is made of Andesite lavas and ashes whwreas the Carn Dearg ridge is granite, with the contact between the andesites and granite approximately following the Allt a Mhuilinn. Granite typically has deep fractures which have suffered alteration during its slow cooling history buried deep in the Earth. These joints control the erosion style of the Carn Dearg ridge. Think of the tors of Dartmoor or the cairngorms, or even the Fiacaill ridge. By contrast andesite is typically highly compact and well bonded so erosion on the North Face is controlled by the vagaries of freeze-thaw and ice plucking making a more rugged and featured rock architecture.