Irish Sea Glacier Lobe v Celtic Sea Piedmont Glacier

Have a look at these two maps.  They show two fundamentally different scenarios for the Devensian maximum glaciation -- one showing Irish Sea ice flowing in a long narrow lobe from NNE towards SSW -- starting in St George's Channel and extending out into the Atlantic approaches.  That's the scenario accepted by many senior UK geomorphologists today.  As I have argued before, a lobe of this shape is inherently implausible because ice does not tend to flow in long thin lobes when there are no topographic constraints -- it tends to spread out into wide fan-shaped lobes instead.  Also, I cannot see how an ice margin very close to present-day sea-level in the Scilly Islands (if the C14 dates are to be believed) can have also been very close to the same altitude in west Pembrokeshire, more than 200 km away.  that's not just a flattish gradient -- it's a zero gradient.  And however much we might argue about basal shear stresses and saturated sediments on the glacier bed, the basic rule is that a glacier will only maintain forward momentum if it has a continuous surface gradient which accords with the direction of ice movement.

So how do we get an ice edge close to present-day sea-level all the way from Pembrokeshire to the Scilly Isles?  The only way, as far as I can see, is to have the ice gradient sloping down from NW towards SE -- which means powerful ice coming off the SE coast of Ireland, thick enough and active enough to "block off" the ice coming down from the N or NE from the Irish Sea and across Cardigan Bay.  There will also have been contact with Welsh ice, coming down from the uplands of mid and south Wales.  This will give us a very confusing picture in Carmarthen Bay, the middle reaches of the Bristol Channel, and down along the coasts of Devon and Cornwall.  The islands like Caldey and Lundy may be crucial in sorting out exactly what happened.......  and we also need to examine the uplands and coasts of SE Ireland to determine whether the ice was really thick enough to sustain movement right across these Celtic Sea approaches and all the way across to the SW coasts of Wales and England.

The outer limit of Devensian ice shown on the bottom map, from Glamorgan to Devon and Cornwall, may be anything up to 50 km adrift in places.   The field evidence needs to be reexamined in order to sort out its precise position.