THE MISSISSIPPI DELTA 



6ii 



consist of a stiff blue clay not unlike the stiff "Port Hudson clays" 

 (Hilgard-29) which underlie the whole delta. Such mud is brought 

 to the surface in a series of mud volcanoes or mud-lumps which, 

 from time to time, arise on the delta surface. These are believed by 

 Hilgard to be formed of the fine mud brought down by the river 

 and precipitated outside of the delta by flocculation. Over this 

 liquid mud are spread the river sediments, the weight of which and 

 that of the growing marshes and their pressure upon the mud 



Fig. 126. The Mississippi Delta. At the "bird foot" the passes are from 

 right to left: North Pass, Northeast Pass (these two unite to 

 form the Pass a I'Outre), South Pass, Southwest Pass. The 

 Northeast Pass has a small southern branch, the Southeast Pass 

 (not shown), while another group of channels, diverges from the 

 "neck"' about five miles above the "head of the passes.'' 



layers result in local upheavals and formation of mud lumps or 

 craters, as long ago suggested by Lyell. Rod soundings in such 

 a crater have reached a depth of 24 feet, but no solid bottom. 

 The rnud flow from these craters varies with the stages of the river. 

 becoming much more lively in times of flood, when great masses of 

 water, or of silt brought down by the river, press upon the layer 

 of liquid mud. Hilgard believes that the banks of the neck and the 

 passes are formed of the disintegrated and redeposited mud from 

 such mud kmips, and that when a mud lump arises in the channel, 

 as has recently occurred, a division is likely to take place. Hilgard 



