206 Greenwood — On Valley Terraces. 



Eivers" (page 152), of the yalley of the Ganges : " In the raw?/ season 

 there is, perhaps, a body of surface water which flows down tlie vale 

 to the sea in volume fifteen times as great as the spring water ; and 

 were every spring of the Ganges permanently dried up, the vale 

 would still be flooded every year by a stream in volume only less by 

 one fifteenth part than that which flows every rainy season now, and 

 fourteen times greater than that which flows in the three dry months." 

 And what makes the river flood in the rainy season except the rain 9 

 " Quum fera dilivies quietos irritat amnes.'" 



And what is even the spring water of rivers but rain reappearing and 

 returning to the sea? Also (op. cit, page 155), "The source of the 

 valley (that is the rain source of the valley) is always much higher up 

 than the source of the river {i.e., than the spring source of the river). 

 The river has no power of making a valley above it, but a torrent of 

 rain water has the power of scooping a valley below it. Terraces of 

 gravel on the sides of chalk valleys are in general remains of beds of 

 rivers. The rivers having eroded the softer chalk sides of their 

 beds, deepen their valleys, and leave their beds as terraces on the 

 hill sides. So that any number of long lines of gravel terraces, 

 which were the beds of rivers or even the beds of rain valleys, may 

 be deposited on the sides of valleys, — first on one side of the valley, 

 then on the opposite side. Such terraces are, usually, on one side 

 of the valley only. Parallel terraces, one on each side of the valley, 

 may be ancient shores. But the vast majority of them are the 

 remains of alluviums where no lakes have ever been. The alluviums 

 are formed by the stoppage of the lowering of the valley. The 

 valley above the stoppage is then worn horizontal. The rain flood 

 waters from the sides and inclined parts of the valley, checked at the 

 flat plains, overflow and deposit alluviums on these flats. The sea 

 stops the lowering of every valley, therefore the bed of every valley 

 is flat and alluvial at the end next the sea. But besides this marine 

 or main alluvial plain, a valley may have any number of what I have 

 called patches of alluvial plain at any distance from the sea. These 

 patches, as I have had the honour to say in the Geological Maga- 

 ziNE,i occur above every hard stratum which crosses the course of 

 the valley or river, and which stops for a time the lowering of the 

 vaUey or river bed. Eain and the river then form a horizontal flat 

 on the comparatively soft strata above the gorge, and an alluvium on 

 the flat. When the gorge is deepened and widened, the alluvium 

 above is cut through, and disappears, leaving only two parallel terraces, 

 and a new flat and a new alluvium are begun at a lower level. Lord 

 Milton gives a drawing of the parallel terraces on the Eraser river 

 which illustrates this theory (see Plate X.).^ The terrace next the 

 river is now in actual formation from the annual overflow of the 

 river and the run of the sides of the valley and the old terraces. 



1 Geol. Mag., Vol. III. p. 519. 



2 The accompanying engraving has been most obligingly lent for the illustration of 

 Colonel Greenwood's paper by Viscount Milton and Dr. W. B. Cheadle. It is one 

 of the numerous illustrations to their interesting work, entitled, " The North -West 

 Passage by Land. Being a narrative of an Expedition from the Atlantic to the 

 Pacific." 8yo. London, 1865. Published by Messrs. Cassell, Petter, and Galpin. 



