554 W. B. Wright— A Theory of the Marginal Drift. 



The belt thus gently invaded with progressively weaker motion 

 and as gently exjDosed without ablation by the slow downward 

 wasting of the dead ice is the older and featureless marginal drift. 

 The line of balance to which the ice retreated in the case of the 

 European Ice-sheet is somewhere in the neighbourhood of the 

 " Young End-moraine ", perhaps a little to the south of it. The exact 

 line was probably the northern limit of the loess, which, except in 

 a few places, does not seem to reach right up to the moraine. It 

 seems natural to suppose that the loess which covers the marginal 

 drift was a product of the dry climate of the maximum of cold, and 

 ceased as soon as there occurred a rise in temperature of sufficient 

 magnitude to introduce the factors of ablation and more copious 

 precipitation. 



The principle of accumulated or residual gradient invoked to 

 explain the advance beyond the line of balance requires further 

 comment, because it is not clear that the ice-motion would not in 

 the main have discharged the gradient concurrently with the 

 establishment of the balance. Two considerations bearing upon this 

 point are worthy of note. In the first place ice-motion must 

 clearly be greatly slowed down as a result of extreme cold, for it 

 depends entirely on the phenomenon of regelation, which becomes 

 increasingly difficult with lowering of temperature. Though the 

 exact references have escaped his memory, the writer believes that 

 Shackleton and Scott have both commented on this checking of 

 glacial movement by cold as being a fact within their experience. 

 This conception, however, when closely examined, though it makes 

 the probabihty of a residual gradient greater, does not settle the 

 question. There is, however, another means of expansion, which 

 probably becomes of great relative importance under conditions of 

 extrenie cold. This is the snow-drift on the surface of the ice-sheet 

 caused by the radial winds. Peary considers that in the north of 

 Greenland this is the controlling factor in distribution. Provided 

 the ice-margin does not extend into the zone of ablation, it is clear 

 that it may be considerably advanced by this method even after the 

 establishment of the balance. What is not clear is that this mode 

 of expansion acting alone would result in sufficient ice-motion at 

 the sole of the sheet to produce even the feebly developed marginal 

 drifts. A combination of both agencies of advance would meet the 

 facts admirably, and account not only for the gradual attenuation of 

 the drift at the feather-edge, but also for such " extra-glacial " 

 phenomena as the " trail " of Southern England. 



Absence of" Marginal Drift " in the neighbourhood of the Atlantic. — 

 It is a noteworthy fact that the belts of marginal, featureless, and 

 loess-covered drift in Europe and America do not extend to the shores 

 of the Atlantic, but in these areas the strongly featured loess-free 

 drift stretches south to the line of maximum glaciation. This result 

 is deducible from the idea that the marginal drift is the product of an 

 advanceduetoresidualgradientandwind-driftaftertheestabUshment 



