EXCAVATION BY MOUNTAIN STREAMS. 153 



are in the Plymouth limestone, as at Brixham and Kent's Hole ; in 

 the Carboniferous limestone, as in the Mendip Hills, Derbyshire, and 

 the district about Settle ; and in the Coralline Oolite of Yorkshire at 

 Kirkdale. 



Streams which flow under ground, like the Mole in Surrey, which 

 flows through the Chalk, traverse chambers which are still concealed. 



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Descending Streams and Rivers. 



The wasting effects of the atmosphere are sensible in all regions, 

 and therefore in every country some materials are available for the 

 streams to transport. But the proportion of matter thus prepared in 

 mountainous countries is so vastly greater than elsewhere, that in 

 general, the less conspicuous effects of the same causes are overlooked 

 in lower regions. The common notion respecting the action of alpine 

 streams appears to be, that these are the principal agents of destruction 

 upon the faces of the mountains, and that it is to them that the actual 

 waste of the surface is to be attributed. But though these streams 

 are indeed powerful agents of excavation, their principal influence is of 

 quite another kind, and it is chiefly by the disposition of the materials 

 brought into them by rains and avalanches that they effect such 

 important changes. 



Erosive or Excavating Action of Streams. In considering the 

 action of streams and rivers, we must distinguish between their powers 

 of eroding or excavating, and of transporting solid matter. 



The river works upon the channel and floodway, and its effects 

 have relation to the consolidation of the matter traversed, and to the 

 rapidity and volume of the moving water. About their sources, 

 and for a long part of their early courses, streams continually deepen 

 their channels, and wear away barriers of rock ; but in their broad 

 expansions near the sea, this power of excavation wholly ceases, as 

 a general law, and is only seen in particular cases, as when great bends 

 are cut off or banks undermined. 



We have abundance of examples in all our mountain regions of 

 the actual excavation of their channels by rivulets and rivers. In 

 the district of Aldstone Moor, the south Tyne runs to the north from 

 the side of Cross Fell, for some miles along a slope of shale, over the 

 Tyne bottom limestone. In this shale, which is itself excavated into 

 a broad valley, the river has evidently cut its own narrow yet suffi- 

 cient channel ; and no contrast can be more striking than that here 

 afforded by the mighty valley of Tynedale, 1500 or 2000 feet below 

 its bordering mountains, and the little channel holding the waters of 

 the river Tyne. Every river works out its own channel in elevated 

 regions, and in lower ground the soft clays and sands yield a passage 

 to the feebler currents. In the level regions, along the rivers of 

 Yorkshire, Lincolnshire, and Cambridgeshire, the channels have been 

 many times changed, even by those sluggish streams ; and still more 

 in the deltas of the Rhine, the Nile, and the Mississippi ; and among 

 the Alps, fluctuation of the river courses is excessively irregular. 



