50 LAKES OF NORTH AMERICA. 



condition. About the margins of these deltas there are small gravel bars 

 that are frequently looped and enclose lagoons. An active struggle is 

 there in progress between the outflowing streams and the shore currents, 

 which has modified the form of the deltas in the peculiar way just 

 referred to. 



A delta advances as fresh material is added to its outer margin, and 

 at the same time the apex of the pile rises and slowly migrates up stream. 

 Such a deposit has a well-defined structure, due to its mode of growth. 

 A radial section made from its apex to any point on its periphery would 

 show three divisions, as is indicated in the following sketch section of a 

 delta built in Lake Bonneville, at Logan, Utah. 



* Fig. 7. — Sectiox of a Delta. 



The history to be read in such a section is this : the fine, evenly strati- 

 fied beds beneath the coarse inclined layers are sediments deposited on 

 the lake bottom, but about the margins of deltas they are usually thicker 

 than on neighboring lakeward areas, owing to more rapid depositions 

 from the waters of the delta-forming stream. In some instances a broad, 

 low apron-like deposit of fine sediment is formed about the lakeward 

 margin of the delta proper. As the coarser portion of a delta increases, it 

 advances lakeward and covers the layers of fine sediment previously laid 

 down, and frequently causes them to become folded and wrinkled and 

 occasionally broken and faulted, on account of the weight of material 

 imposed upon them. 



The boulders, gravel and sand brought down by a stream are carried 

 to the outer margin of its delta, and roll and slide down its submerged 

 lakeward slope so as to form inclined layers. The angle of inclination 

 of these layers is the angle of stability in water of the material forming 

 them. Where the deposit is mainly of rounded stone and gravel, the 

 angle of slope is in the neighborhood of 30 to 35 degrees, but in some 

 instances is steeper and the structures are unstable and favorable for 

 landslides. 



The triangular area shown in the section, above the inclined beds, is 

 the subaerial portion of the delta, built by the stream in meandering 



