PERCHED BLOCKS AND ASSOCIATED PHENOMENA. 535 



has attained such a height that the rain can beat in under the 

 boulder, and grass, which would not grow where there was no 

 moisture, can creep up to the base, the pedestal gets eaten away 

 at its base (probably more on one side than another according 

 to the prevalent winds), and the boulder topples over. So such 

 perched blocks may represent only a stage in the process of denu- 

 dation, and the approximate uniformity of height of the pedestals 

 may depend upon other circumstances than the time during which 

 they have been exposed. 



The pedestals are of the same height only over limited areas ; and 

 that has been much helped by the fact that, in each case, they 

 almost exactly represent the thickness of beds which have readily 

 peeled off. The true pedestal, due directly to the protection of 

 the boulder, is often determined by a bed only a few inches in 

 thickness, while there is generally another similar bedding-plane 

 along which the more rapid denudation accompanying the encroaching 

 grass and other plants has acted and which, arrested at the boulder, 

 gives the impression that the true pedestals are higher and more 

 regular than they really are. 



Next, with regard to the mode of transport of the boulders, it 

 must be remembered that they have all obviously travelled in the 

 direction of the furrows on the rock on which they rest, from rock 

 in place close by to the north of them. They are perched in such 

 positions and at such levels as would be difficult to explain on the 

 supposition that they were brought there by floating ice drifting 

 in the direction of the furrows and grounding ; and it would be 

 impossible to imagine that, during the period of emergence, the 

 glaciated surface of the rock could have escaped the wasting action 

 of the waves, while marine currents of no great force would cer- 

 tainly have trundled such boulders along the smooth rock-face and 

 swept them into the valley below. 



Nor can the blocks be the heavy remainder of a mass of Boulder- 

 clay, the finer part of which has been removed by denudation ; for 

 the blocks, unlike the varied constituents of the nearest drift, are all 

 fragments of one and the same rock, though the ice must have 

 travelled over a great variety of well-marked formations, all of 

 which are represented in the Boulder-clay. « 



Here and there, on Farleton for instance, there are a few stray 

 stones of much smaller size than the average pedestal hlocJcs and 

 derived from a different source. These may represent some of the 

 regular Boulder-clay not quite scraped off by the last advance of the 

 ice. 



Had the drift been removed by the action of water, whether of 

 streams or the sea, so that there was no clay left under and around 

 the large boulders, the rock under them must have been water-worn ; 

 while if there was Boulder-clay round and under them, some would 

 probably be preserved there now. Besides this, had the clay once 

 extended around the boulders, it would have collected the rain-water 

 into rivulets, which would have get under the boulders and destroyed 

 the surface of the limestone. 



2o2 



