6i6 PRINCIPLES OF STRATIGRAPHY 



Cementation of Delta Deposits. The delta deposits may be 

 compacted merely by pressure, or the component particles may be 

 bound together by the introduction of a cement. The Rhone delta 

 contains much sandstone, the grains of quartz being bound by a 

 calcareous cement, due to the abundance of lime in the stream. 

 The presence of numerous limestone pebbles in glacial deltas also 

 becomes a source of lime which is redeposited among the pebbles 

 and cements them. Such an example is found in the partly con- 

 solidated gravels of the Pleistocenic delta in the Ontario basin near 

 Lewiston, and a still more extensive one in the Nagelfluh delta of 

 the Salzburg region already referred to. Cementation by infiltrated 

 iron oxides also occurs. 



Deposits of lime are especially abundant in deltas of arid 

 regions, such as the Volga, the Indus, the Nile, and the Colorado. 

 Extensive deposits of massive travertine and caliche have been 

 formed in Arizona and New Mexico. Such deposits are, how- 

 ever, also found where the river water is high in lime content, as 

 in the Rhine delta and in the Rhone delta already mentioned. In 

 the latter case the approach to a semi-arid climate over the delta 

 in the mediterranean is a further factor aiding deposition of lime. 

 The silt of the Rhine delta in the Lake of Constance contains 

 30.76% of CaC03, 1.24% MgCO;,, and 5.20% FeCOg. The other 

 principal constituents are SiOo 50.14%, ALO3 4.77%, Fe.Og 2.69%, 

 and small quantities of the oxides of manganese, magnesium, cal- 

 cium, potassium, and sodium. 



Modification of the Delta Surfaces. The chief modifications 

 sufifered by the delta are wave and wind erosion, and the forma- 

 tion of deposits over the surface of the delta. Wind erosion affects 

 the surface of the delta, while wave erosion occurs around the 

 margin of the delta when the force of the river no longer is able 

 to continue construction. It is common on a sinking coast, as 

 is shown by the encroachment of the sea on the deltas bordering the 

 North Sea. Wave erosion may also become effective when delta- 

 building has practically come to an end owing to the decrease of 

 supply incident to development of low relief with old age. In such 

 a case the sea may gradually encroach on the delta and plane away 

 the upper layers, until the remaining remnant of the delta is wholly 

 submerged, when normal marine sediments may succeed. In all 

 such cases there is commonly found a fringing belt of dunes at 

 the outer margin of the delta, as in the case of the Rhine delta. 

 Such dunes may also be formed where building is still in progress, 

 as in the Rhone delta, where dunes are formed between the two 

 principal mouths of the stream. Other eolian deposits may form 



