306 Ooldthwait — Twenty-Foot Terrace and 



doubtful if a mature outline and profile could be readied before 

 the relative level of laud and sea would be changed by crustal 

 deformation. 



In seeking to interpret the significance of the Micmac terrace 

 it is important to bear in mind its uniform altitude between 

 Quebec and Matane. The approximate horizontally of the 

 shoreline for 225 miles precludes the view that there has been 

 a local change in high-tide level.* A change of twenty feet 

 near the head of the estuary would be accompanied by a rela- 

 tively small change of level out near the Gulf. Moreover, the 

 configuration of the coast, near Gaspe, gives no suggestion of 

 any barrier which could have lowered the high-tide mark 

 twenty feet in recent times. 



A factor in cliff development which is rather seldom empha- 

 sized is slow coastal subsidence. Its importance was recognized 

 and clearly stated by Dr. Gilbert in 1883 f : 



" A slow and gradual submergence modifies the products of 

 littoral action. The erosion of sea-cliffs is exceptionally rapid, 

 because the gradually deepening water upon the wave-cut ter- 

 races relieves the waves from the task of carving the terraces, and 

 enables them to spend their full force against the cliffs. The cliffs 

 are thus beaten back before the advancing tide, and their precip- 

 itous character is maintained with constant change of position." 



The truth of this statement, while sufficiently supported by 

 Doctor Gilbert's masterful analysis of the physics of wave work, 

 is also confirmed by observation. It is not unlikely that 

 Doctor Gilbert had slow submergence in mind as one, at least, 

 of the reasons for the marked cliff cutting at the high-water 

 marks of the extinct Lake Bonneville. In the Great Lakes 

 region the exceptional strength of the JNTipissing shoreline and 

 its peculiar cliff and terrace profile have been attributed, not 

 simply to the greater duration of that stage, but to the fact 

 that it was preceded by a prolonged submergence, due to the 

 slow uplift of the Nipissing pass, which was then the outlet of 

 the lakes. Certainly, if on the one hand slow submergence 

 favors cliff cutting, while on the other hand slow emergence 

 binders it. and favors beach building, there is no lack of illus- 

 trative material among the extinct shorelines of the Great 

 Lakes region, where beach ridges commonly mark halts during 

 emergence, and extensive cliffs characterize the stages of climax 

 during submergence.^ 



* See papers by J. W. Dawson, referred to in later notes, especially : On a 

 modern submerged forest at Fort Lawrence, Nova Scotia. Quart. Journ. 

 Geol. Soc, vol. xi, pp. 121-122, 1855. 



f G. K. Gilbert : The Topographic Features of Lake Shores ; Fifth Annual 

 Report of the U. S. Geological Survey, 1883-1884, pp. 67-123, and especially 

 pp. 110-111. 



fj. W. Goldthwait : Physical Geography of the Evanston Waukegan 

 Region ; Illinois State Geological Survey, Bull. No. 7, 1908, pp. 66-68, and 

 figure 29. 



