Sea-Cliff of the Lower Saint Lawrence. 307 



It may confidently be said, therefore, that while a mature 

 cliffed shore will result from prolonged wave action on any 

 coast, the process will be greatly accelerated if that coast is 

 sinking. 



Hypothesis of an Interval of Stability between Two Uplifts. 



With the foregoing principles of shoreline morphology in 

 mind, we may frame a hypothesis for the development of the 

 Mictnac terrace and sea-cliff, which will be as follows : After 

 the shores of the Saint Lawrence estuary had risen differen- 

 tially, without interruption, a few hundred feet out of the 

 Champlain sea, uplift ceased, and a long period of stability 

 ensued. During this period, which we may call the Micmac 

 stage, the irregular shoreline was straightened, its profile was 

 corrected, and a line of cliffs were developed and cut back two 

 or three miles towards the interior. At length, after this pro- 

 digious cliff recession had been accomplished, a second uplift 

 occurred. This may have been a slow, epeirogenic movement, 

 like the first one ; or, perhaps, a sudden, seismic movement. 

 If epeirogenic, it may still be in progress. 



Examining the facts more closely, with this hypothesis in 

 mind, we may test the working value of it. 



The length of time required for the cutting back of the cliffs 

 so far into the land is very great, in comparison with the time 

 required to build any one of the weak upper beaches. One of 

 these might have been constructed by a single storm during 

 the period of emergence ; the Micmac shoreline, however, with 

 its wide off-shore terrace, plainly represents centuries of littoral 

 marine planation. If we take the modern shore of Lake Mich- 

 igan as similar in character to the Micmac shore, and assume 

 an average rate of recession of the Micmac cliffs of 5 feet per 

 year,* and a total recession, as at Isle Verte and Trois Pistoles 

 (see figure 3, A and B), of 3 miles, the Micmac stage lasted 

 somewhat more than 3000 years. This is a reasonably small 

 fraction of post-glacial time, as inferred from the history of 



* The recession of the high clay cliffs on the west side of Lake Michigan 

 between Milwaukee and Evanston, as computed in 1868 by Dr. Edmund 

 Andrews, was about 5 feet per year. See his " North American Lakes con- 

 sidered as Chronometers of Post-glacial Time," Transactions of the Chicago 

 Academy of Sciences, vol. ii, pp. 1-23, 1870 (quoted by Frank Leverett, in 

 " The Illinoian Glacial Lobe," Monograph No. 38 of the U. S. Geological 

 Survey, 1899, pp. 456-457). While the height of these modern lake cliffs is 

 about the same as the height of the Micmac cliffs, and the structure of the 

 glacial drift which composes them is essentially the same also, the terrace of 

 the former has a width of barely two miles instead of three or four. It 

 seems fair, therefore, to regard the greater width of the Micmac terrace as 

 compensating for the greater vigor of the waves in the estuary, if, indeed, 

 the wave action in the Saint Lawrence is any stronger. 



