132 



PHYSICAL GEOLOGY 



head of the delta of the Hoang Ho is 350 miles from the coast. 

 The Imperial valley in California is the result of delta building. The 

 Gulf of California formerly extended 150 miles further northwest 

 than now, and across it a delta was built by the Colorado River, so 

 high as to shut off the upper part of the gulf and inclose a lake of 

 salt water. This lake has almost entirely disappeared and its bed 



[22. — Longitudinal section of a delta, showing the dipping, fore-set beds. 

 (Photo. R. S. Tarr.) 



has become the Salton sink. Thanks to irrigation this basin is ex- 

 tremely fertile. 



The depth of delta deposits is often great. A boring at New Or- 

 leans encountered driftwood at 1042 feet, and depths of 500 feet 

 are not uncommon in other deltas. It has been shown that in many 

 cases the subsiding of deltas progresses at a pace about equal to the 

 deposition. 



Deltas are usually noted for their fertility. The three most 

 densely populated regions of the world, outside of cities, are the 

 deltas of eastern China, India, and the Po River in Italy. This is 

 true in spite of the fact that, because of their level surfaces, deltas 

 are especially subject to floods. The great flood in the Mississippi 

 River delta in 191 2 destroyed many lives and millions of dollars' worth 

 of property, and this was also the case with earlier floods. One of 

 the notable examples of such easily flooded districts is the delta of 



