192 THE BOOK OF NATURE STUDY 



the river are not such as will shift the deposit into deeper waters, 

 the material gradually accumulates and forms new land, so that 

 the river mouth is always moving seaward. Along the Channel 

 and on the east coast of England, most noticeably in the Wash, 

 the land is always steadily gaining on the sea by means of the 

 detritus washed down by the rivers. 



Some of the material, however, does not reach the sea, but 

 accumulates along the river course ; nearly all rivers, as soon as 

 their early mountainous rush is over, will be found flowing quietly 

 between broad level meadows, which are apt to be covered from 

 time to time with flood water. On examining the soil beneath 

 one of these meadows it will be found to be practically identical 

 with the silt deposit which had previously settled out from the 

 glassful of turbid river water. Careful examination of the 

 meadow will also as a rule show that its level rises very slightly 

 towards the water's edge, the actual river bank being generally 

 elevated a few feet farther. In consequence, when a flood occurs 

 and the turbid flood water invades the meadows, it is there some- 

 what pounded up, and does not simply flow back into the river 

 when its level falls. Instead the flood water sinks through the 

 soil or oozes back under ground into the river bed ; but in so doing 

 it leaves behind on the surface of the meadow the load of silt it 

 was carrying before. After a flood has subsided the grass of the 

 river meadows will be found all sticky and muddy, and the whole 

 surface of the meadow has really been raised to a very small extent 

 by the deposit left behind. Thin as the layer may seem, the whole 

 soil below the water meadow has been deposited in this way ; 

 it is made up of earth washed down from some district higher up 

 the river's course, flood after flood it has grown and thickened 

 until it has gradually spread across the valley. The river may 

 still be seen cutting away its banks in places and removing what it 

 has previously deposited, but provided that the slope of the valley 

 is getting flatter so that the river is decreasing in velocity as it 

 gets nearer the sea, it will always be leaving behind more than it 

 takes away and gradually increasing the thickness of the soil over 

 the stretch of meadows. Because the river sediment, however, has 

 been derived from rocks higher up in the valley, and because it has 

 been subjected to a certain amount of sorting by the running water, 



