DELTA STRUCTURE. 



97 



acquires a more or less well-defined oblique stratification, which mny be 

 called a "delta structure." During the building of a delta the stream mean- 

 ders over all parts of its surface that ai-e not submerged, and, in irregular 

 succession, discharges at all points of its periphei'y; in this manner the 

 stream-borne debris is carried to all points on the edge of the deposit and 

 allowed to roll down the submerged slope. The strata thus formed are in- 

 clined, the amount of their inclination depending upon the angle of stability 

 in water of the material deposited. Observation has shown that the slope 

 of a delta scarp commonly is from 20 to 25 degrees. The fine sand and. 

 mud held in suspension is cai'ried farther than the stones and gravel, and 

 is deposited about the base of the delta scarp, decreasing in quantity and 

 becoming finer the farther it is carried from the point of discharge. The 

 stream-borne silt thus tends to build up a secondary cone outside the base 

 of the main delta, which, on its outer margin, merges with the lacustral 

 sediments dejoosited in the central portion of the lake. In tlie growth of a 

 delta the scarp of coarse debris is gradually advanced on all sides and con- 

 sequently overplaces the secondary cone at its base; this added weight fre- 

 quently causes the fine sediment to be crumpled into folds and perhaps 

 broken by small faults. 



A delta deposited at the mouth of a high-grade stream will have three 

 well marked divisions, as shown in the following diagram: 



WATER SURF-ACS 





SZSSSS^^l 





Flu. irj.— Ideal section of a high-grade delta. 



At the top is an alluvial cone («) of unassorted material, resting on (6) 

 a deposit of obliquely stratified gravel, which in turn is built out over a 

 secondary cone (crt) of sand and silt. As a delta grows, a becomes thick- 

 ened, and is built out over b, which at the same time is carried forward over 

 c. In this manner the characteristic tripartite structure of deltas originates. 

 In low-grade streams, which all long rivers necessarih- are, the material 

 MoN. XI 7 



