THE TUrOGKAPHY OF LAKK SHOHES. 45 



waves that the h\iul is cut away so as to form sea clift's and terraces. 

 This is shown in a striking manner along large portions of the shores of 

 Lake Superior, where bold cliffs, an inheritance from a previous topo- 

 graphic cycle, })lunge into deep water, and are w^ithout talus slopes or 

 other loose deposits within reach of the waves. In these instances there 

 is scarcely a mark on tlie rocks that would record the present horizon of 

 tlie lake should its waters be witlidrawn. Clear watei's may dissolve the 

 rocks against which they dash, however, and when cliffs of limestone 

 and other easily soluble rock descend into deep waters, a line of grottoes 

 and caves may be formed below the upper w^ave limit, and perhaps increase 

 until a shelf is produced on which sand and pebbles could lodge. When 

 this happens, erosion by solution is assisted by mechanical means, slight at 

 tirst. but increasing as the conditions become more favorable, until cliffs 

 and terraces result- 

 Terraces. — The terraces about the margins of existing lakes are 

 usually covered with the loose stones and sand, and form the beaches on 

 which one may walk during calm weather. 



The surface of a typical lake terrace slopes gently lakeward and is 

 bounded on the landward margin by the upward slope of the accom- 

 panying sea cliff, and on the submerged, lakeward margin by a down- 

 ward slope leading to deeper water. These terraces owe their formation 

 to excavation or to deposition, and in most instances the two processes 

 are combined. Even when the terrace is due principally to excavation, 

 there is a surface layer of rounded debris resting on it, wliich is usually 

 thickest on the lakeward margin and forms the lakeward slo})e. These 

 features are shown in the following cross section of a lake shore, where a 

 compound terrace is being formed, and also on Plate lo. 



Fio. 3. — Profile of a cut ani> hiii.t TKiujAfE. 



On preci})itous, rock v shores, terraces are not prcxbued, for the ivason 

 already stated in considering the nrigiu of sea cliffs, that the debris from 

 the land falls into deep water l)eh>w the I'each of the waves. 



