A HISTORY OF CUMBERLAND 
the core increases, the river-course by degrees merges into line with the 
junction between the inner and the outer rock. Indeed it may finally 
take a very different course from what it had on first reaching the 
enveloping rock. The Lune between Ravenstonedale and Tebay and 
the upper waters of the Lowther affords good examples of these modifi- 
cations. Both began to flow at a very much higher geological horizon, 
and in rocks which have long since wasted entirely away. Both have 
cut their way down into a complex mass of rocks whose weakest direc- 
tions lie transverse to the original course of the stream. Hence the 
present trunk stream of the Lune, which at one time rose on a tributary 
of that river over the summit of the Howgill Fells and flowed westward, 
just on the north side of the line of highest ground there, has gradually 
followed the edge of the Mountain Limestone down hill, as the envelope 
consisting of this rock has gradually wasted from the hard, dome-shaped 
core of greywacke. This explains how it happens that the river flowing 
westward through low ground on the north side of the axis of the Lake 
district abruptly turns to the south at Tebay, and thence cuts its way 
right across a mountain mass, consisting of some of the toughest rocks in 
the kingdom, to the low ground beyond, and flows past Kirkby Lonsdale 
to the sea. 
As this example is typical, and its comprehension involves a reference 
to the mode of attack of rivers in all cases of this kind, a brief explanation 
of the process may be given here: River valleys are wide where the 
waste by atmospheric agencies keeps ahead of the rate at which the river 
cuts down its channel ; and they are narrow where the reverse is the 
case. That is to say a river-channel is usually narrow where the stream 
traverses hard rocks, and wide where it crosses soft. Now a river 
flows at a slower rate through a wide channel than through one that is 
narrow. The Eden for example quietly, almost lazily, eddies its way 
seaward through the soft marls and the alluvium which form the meadow 
land about Lazonby ; but when it arrives at Eden Lacy and finds its channel 
narrowed to the hard rocky gorge formed by the Penrith Sandstone there, 
it seems to wake up and to hurry onward at a rate very different from 
what it had in the wide expanse formed by the softer rocks. From side 
to side the river at this point is not more than two thirds as wide as it 
was a mile above ; hence its swifter flow. Thus the power of running 
water to transport stones, and therefore to wear its river-channel, is 
proportional to the sixth power of its velocity. That is to say, water 
flowing at a rate sufficient to roll a stone a quarter of an ounce in 
weight, will, if its rate of flow is doubled, be able to drift a stone weighing 
a pound, and so on in the same proportion. Where hard rocks form a 
river-bed, and the channel therefore is narrower, the rate of flow of the 
stream is increased, and the river exerts in consequence greater erosive 
power, just at the point where that extra effort is most required if the 
river is to maintain its course. In other words, where impediments are 
placed in their way the streams rise to the occasion and put forth an 
amount of energy sufficient to overcome the obstacle. The beautiful 
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