THE TOPOGRAPHY OF LAKE SHORES. 51 



over its surface. It is really an alluvial cone, similar to the conical piles 

 of debris so common in desert valleys at the mouths of high grade canons. 

 It is irregularly stratified, the layers being inclined at a Ioav angle corre- 

 sponding Avitli the slope of the surface of the structure at the time they 

 ■ were laid down. 



The change from the- gently sloping and irregularly bedded material 

 of the alluvial jiortion or cap of the delta, to the steeply inclined and more 

 regularly bedded layers, marks the level of the lake in which the de]:)osit 

 was formed. The outer margin or periphery of the delta, is in a horizon- 

 tal plane and retains the same position as the delta advances, providing 

 there is practically no change in the level of the lake surface. The surface 

 slope of the cap of the delta, along radial lines from the apex to the peri[)h- 

 ery, is gently concave to the sky. On recent examples the surface is 

 frequently scored with radiating and branching channels, or " distribu- 

 taries," left by the changeable stream that built the structure. As a 

 delta increases in size its apex rises and slowly migrates up stream, as 

 already stated, so that in large deltas of high-grade streams the apex is 

 frec|uently well within the mouth of the canon through which the drain- 

 age is delivered. 



In the deltas of low-grade streams, like the Mississippi, the divisions 

 noted above are not readily distinguishalile, as the material forming them 

 is fine throughout and the inclination of all the layers is gentle. 



Should the surface of a lake be lowered after having stood at a definite 

 horizon for a long period, the terraces, embankinents, deltas, etc., formed 

 about its borders become conspicuous features of the exposed land surface 

 and another series of similar forms is at once begun at a lower level. 

 Should another subsidence follow, another series of horizontal lines will 

 be added to the topography of the shores. A rise of a lake causes 

 the submergence of previously formed shore features, and they may be- 

 come covered with fine sediment or have other wave and current-built 

 structures imposed upon them. Such changes lead to puzzling compli- 

 cations in the records, as has been ol)served in many instances where lake 

 basins have been emptied and their sides and bottoms laid bare. 



Ice-built walls. — In addition to the topographic features character- 

 istic of lake shores thus far noticed, there are others due to the action of 

 ice. In northern latitudes the formation of sea cliffs, terraces, embank- 

 ments, etc., about tlie maigins of lakes, exce[)ting those of large size, 

 takes place mainly in tlie smuinei' season. In wintei'. when most small 



