GEOLOGICAL CLASSIFICATION OF SOILS 47 



lagoons, which are ideal not only for the further deposition 

 of alluvial matter but also for the formation of cumulose soils. 

 This state of meander naturally increases the probability of 

 overflow in high water, a time when the stream is carrying 

 much suspended matter. This suspended material is deposited 

 over the flooded areas; the coarser near the channel, building 

 up natural levees; the finer sediment farther away in the 

 lagoons and slack water. 



Due to a change in grade, a stream may cut down through 

 its already well-formed alluvial deposits, leaving terraces on 

 one or both sides. Often two, or even three, terraces may be 

 detected along a valley, marking a time when the stream-bed 

 was at these elevations. On the lower slopes of hills bordering 

 valleys the colluvial deposits may touch or even mingle with 

 the alluvial, furnishing the stream with some detritus. Flood 

 plain soils are variable in character, ranging from sandy loams 

 to heavy clays. 



A great deal of the sediment carried by streams is not de- 

 posited in the flood plain but is discharged into the body of 

 water to which the stream is tributary. Unless there is suffi- 

 cient current and wave action the suspended material ac- 

 cumulates, forming a delta. Such deposits are by no means 

 universal, being found at the mouths of but a small propor- 

 tion of the rivers of the world. A delta is generally a con- 

 tinuation of the flood plain and as it is built farther and far- 

 ther out the stream is forced to aggrade its bed and both 

 flood plain and delta are raised. Near the front of the delta 

 the land is swampy ; farther back it is higher and may assume 

 considerable agricultural importance. 



Where streams descend from mountains or plateaus, sudden 

 changes in gradient often occur as the stream emerges in the 

 lower lands. A deposition of sediment is thereby forced, giv- 

 ing rise to alluvial fans. 1 They differ from deltas in their 



1 As already noted, alluvial fans and colluvial material are very closely 

 related. Soil survey classifications usually do not recognize the latter 

 separation. 



