180 DALY. 



. level within the tropics are harder to make. They must depend on 

 an assumption as to the life of each full-bodied mass of ice after it has 

 been once formed. That the later ice-sheets long persisted with 

 considerable thickness is shown by the depths of fiord and other basins 

 which have been glacially excavated. The imposing depth and 

 breadth of the Grand Coulee in Washington State, cut by the Columbia 

 river during merely a sub-stage in the last Glacial maximum of the 

 Cordillera, is another ciualitative proof. If the rate of ice movement 

 at the climax of the Wisconsin stage were known, it might be possible 

 to give a minimum estimate for the din-ation of that climax; the data 

 would be found in the distribution of special types of erratics, especially 

 either those derived from high nunataks, or those carried over high 

 masses like the Adirondacks or White Mountains. 



Though the whole subject is very obscure, the general probabilities, 

 viewed in relation to the estimates by Chamberlin and Salisbury, 

 suggest a period of from 50,000 to 200,000 years for the time during 

 which the Pleistocene ice-caps were nearly or quite at their greatest 

 volume. During that total period the tropical ocean had a level lower 

 than now by an amount ranging from 30 m. to 75 m. Then occurred 

 the deeper benching and smoothing of platforms by waves and cur- 

 rents. 



Nevertheless, the sea was actively attacking the islands and conti- 

 nental coasts throughout nearly the whole Glacial period. The reef- 

 building corals were largely killed off long before the ice-caps of the 

 first (xlacial stage reached their full size. The succeeding Inter- 

 glacial stage may have witnessed a partial re-establishment of reefs 

 in the open ocean, but, if so, such reefs must have been relatively 

 feeble and short-lived defenders of the islands. Similar reasoning- 

 applies to the other recognized stages of the Glacial period. Hence, 

 though sea-level swung down and up several times, lively wave al)ra- 

 sion must have been almost continuous. 



Rate of Pleistocene Wave-benching. Unfortunately little is accurately 

 known concerning the speed with which ocean waves can drive in 

 shore cliffs on the deep-sea islands. Most of the measurements so far 

 made refer to coastal points affected only by the less powerful waves 

 of the North Sea, the Mediterranean, or the inner edge of the conti- 

 nental shelf of Europe. The clayey cliffs of Yorkshire, England, 

 retreat at the rate of 2 in. to 4.5 m. per aunum. Matthews states that 

 the shore-lines of Suffolk and Norfolk (also on the east coast) are being 

 driven in at rates of from 3.5 m. to 14 m. per annum, while the rate 

 for the Welsh coast, between Llanelly and Kidwelly (Bristol Channel) 



