4-1 LAKES OF NORTH AMERICA. 



the nomenclature of the t)ceanic shores where topographic forms similar in 

 character and in origin exist in many places on a magnificent scale. Varia- 

 tions in the appearances of sea cliffs in soft and hard material are shown on 

 Plates 8 and 9. These illustrations have been selected from a large num- 

 ber of photographs taken by the writer on the borders of the Laurentian 

 lakes, and illustrate the two types of shore features there most pronounced. 

 The recession of sea cliffs may be best studied when a gale is blowing 

 directly on shore. At such a time, each Avave as it .reaches shallow 

 water and surges up on the land, carries forward the gravel and sand^ 

 within reach and dashes it against the base of the cliff and tends to 

 wear it away. The finer products produced by the friction and pounding 

 of the loose stones against each other and against the cliff, are carried lake- 

 ward by the under-tow, leaving the coarser fragments ready to be caught 

 up by the next iurush of water and the process repeated. As the cliff is 

 under-cut, fresli angular fragments fall from its face to the beach below 

 and are at once attacked by the waves and sooner or later reduced to 

 rounded gravel and sand. The cliff thus furnishes the tools for its own 

 destruction. ^ 



The manner in which lakes 



^^^ wear away the land confining 



,,-^p^ them is illustrated in the fol- 



.J:fjrf-._'?^f.'T'ff_€ ^^^^^P> lowing section of a rocky shore, 



^^^^^^ which also shows the relation 



^^^^^^^^^^^P^ of the sea cliff 5 c to the 



platform or terrace « c at its 



Fig. 2. —Profile of a Sea Cliff and Terrace. , 



base. 



Waves are only able to reach the land in a narrow vertical interval, 

 determined mainly by the difference in their height during calm weather 

 and when storms are raging. Even in the case of large lakes this inter- 

 val does not exceed ten or fifteen feet, and on account of the debris 

 usually encumbering the shore, the actual zone of erosion on the fresh 

 rock surface is normally very much less than this. The waves thus act 

 like a horizontal saw cutting into the land. The result is that at the 

 base of every sea cliff there is a platform or terrace, as indicated in the 

 above diagram. The junction of the sea cliff with its accompanying ter- 

 race is a horizontal line, determined by the elevation of the lake surface. 



Lake waters unaided b}^ debris, like the waters of clear streams, have 

 but slight power to erode. It is only when the margin of a lake is suffi- 

 ciently shallow to Ijring the debris on its bottom within the reach of the 



