204 



GEOLOGY. 



to the strength, direction, and relations of waves and currents. The 

 consideration of these forms belongs more properly to the work of the 

 sea than to that of rivers, since rivers are not concerned in their con- 

 struction except in supplying material. 



Delta lakes. — Delta-building streams sometimes help to form 

 lakes by throwing their deposits around an area which fails to be 

 aggraded to sea-level. Lake Pontchartrain, and other lakes in the 

 delta of the Mississippi are examples (Fig. 187). 



Fig. 192. — Terraces of the Frazier River at Lilloet, B. C. (Calvin.) 



STREAM TERRACES. 



Stream terraces ^ are bench-Hke flats or narrow plains along the sides 

 of valleys (Fig. 192). They are usually narrow, but sometimes have 

 great length in the direction of the axis of the valley. They originate 

 in various ways. 



Due to inequalities of hardness. ■ — Reference has already been 

 made (p. 140) to the effect of hard horizontal layers in the development 

 of terraces and terraciform projections on the sides of valleys (Fig. 120). 

 Such terraces are the result of differential degradation, and the upper 



* For discussions of terraces see Gilbert's Henry Mountains, p. 126; Davis' 

 River Terraces in New England, Bull, of the Mus. of Comp. Zool., Geol. Series, Vol. 

 V, pp. 282-346 J and Dodge, Proc Boston Soc. of Nat. Hist., Vol. XXVI, pp. 257-73. 



