RIVER VALLEYS. 141 



Valleys in Low Plains. But while the tableland was in process 

 of being elevated and the mountain-peaks were being denuded 

 from off it, the rolled and worn roek fragments thus formed were 

 removed by tidal waters from the heights into the adjacent syn- 

 clinal depression, as elevation went on. As this detritus increased in 

 amount, the force of the waves carried it lower and lower and spread 

 it out evenly ; and when eventually this low plain rose from the sea, 

 the waters draining down from the higher lands ploughed out a course 

 over it, which is the simplest form of a river valley. Such valleys 

 cut through the pampas-mud are typified by the channels of the La 

 Plata, the Amazon, the Orinoco, and most of the rivers of South 

 America. Something of the same sort of thing may be seen in the 

 rivers of Siberia, which flow through a deposit of mud worn from off 

 the plain of Thibet and the northern mountain chains of Asia, by a 

 sea which has retired. In our own country something of the same 

 kind, but on a small scale, is seen in the plain called the Carse of 

 Stirling, which is a silt washed down from the hills of Stirling and 

 Perth, through which the Forth passes to the sea. 



But river valleys have generally been formed gradually during 

 long periods of time, and have been excavated far more largely by the 

 power of the sea than by the rivers themselves. When once a depres- 

 sion exists by which tidal waters make their way into the land, the 

 rising and falling of the tide acts twice a day like a saw on the shores 

 of the estuary and the river-banks, so as to waste the rocks and widen 

 the channel. And when a land drained by rivers is slowly depressed 

 in level, the sea is given an entrance further into the land, and 

 so the tide widens the river valley at that point in the same way 

 as it was widened at the river's original mouth. Thus by depres- 

 sion estuaries enter the land, and carve out channels which vary 

 in breadth and depth, partly with the nature of the rocks, partly 

 with the angle of tbeir upheaval, and which are considerably influ- 

 enced by the rate at which movements of the earth's surface go 

 on. And at last the sea, covering broad tracts, rounds oif the rough- 

 nesses of its work by tidal movement in waters of moderate depth, 

 so as to take away the abrupt characters of the cliffs in the gorges 

 thus excavated. Upheaval causes the land to emerge again in the 

 same slow way as it was depressed, only with this difference, that 

 much of the fine sediment denuded is carried away to the ocean, and 

 the chief part of the detritus already accumulated will be swept out 

 from the valley, so as to leave, when the waters retire, beds of gravel 

 and of inundation mud upon what had formerly been an estuarine 

 sea-bed. And when the emergence of the land is completed, a long 

 and comparatively broad and shallow valley remains, with broad branch- 

 ing tributaries, at the bottom of which the river runs. Such a 

 channel is the. valley of the Thames, in the lower part of its course. 

 Oftentimes the sediment swept out from a river valley by the sea, 

 when the land was lower, remains at the mouth of the river as a delta, 

 or has constituted the obstacle which caused a delta to be formed 



Synclinal Valleys. There are two other kinds of valleys which, 



