CHAPTER XI 



DEPOSITION 



DUST and mud worn from a road are found to be carried by 

 the rain torrents so long as the latter have sufficient velocity. 

 If the water passes off to a drain or brook the debris is carried 

 away still farther by them. But if hollows occur in the road 

 surface, or if the gradient flattens out, some or all of the debris 

 will be deposited, and the deposit may be examined, when it has 

 dried somewhat, by cutting across it with a knife or spade. The 

 nature of the material and its arrangement are the two points 

 to which attention should be especially directed. Coarser matter 

 will have been the first to drop, then finer sand, and at last the 

 finest grained mud, the position of each being related to the 

 slackening velocity of the water current. Thus deposition is 

 accompanied by sorting. The arrangement of individual frag- 

 ments, such as water-logged leaves and sticks and bits of slate or 

 shale, will show that they settle with their flat sides horizontal, 

 and their long axes in the direction in which the stream was flowing. 

 Some of the debris swept down by a stream will be found de- 

 posited wherever the stream slackens in velocity. If it has over- 

 flowed its banks it will be found to have spread an even coat of 

 mud or sand over the flooded meadows, laying down a thin sheet of 

 sediment over the whole area, which in these cases is generally a flat 

 one (Fig. 68). Digging down below the surface, it may be possible 

 to show that the soil in which the meadow grass was growing before 

 the flood is made of earlier deposits of the same kind of stuff 

 laid down in similar flat layers. If we imagine the same process 

 to have gone on for a long period, a great thickness of silt will be 

 thus laid down in successive sheets parallel to one another and to 

 the surface on which they began to be laid down. Such thin sheets 

 are called lamince, and the structure as a whole lamination of the 

 deposit. 



