538 Trof. W. Keeping — Geology of Aberystwyth. 



their long axes arranged parallel with the surface of the slate. 

 Those great forces which puckered up the rocks into such extra- 

 ordinary contortions, also caused that arrangement of the particles 

 of rock in virtue of which the rock now splits or " cleaves " into 

 thin slates.^ 



It is reasonable to expect that some of the transition stages 

 betyveen the old sea muds and the well-formed slates should in 

 some places be found ; and Aberystwyth is such a place. There is 

 no good slate within some few miles of the town, but the clay 

 group of rocks is represented by irregular shale, rubbly rab, soft 

 slate, and "bastard" slate ; very commonly however traces of cleavage 

 are seen in the shale more or less clearly marked, and " rab " is in 

 many cases produced by cleavage, often combined with joints and 

 bedding planes. 



Along the shore near Clarach well-marked slate may be seen. It 

 is here soft and useless, but, nevertheless, it is true slate, for it may 

 readily be seen that the lines of division are thin, and regular in 

 direction, and that they cut across the planes of stratification at a 

 high angle. 



The constancy of the direction of the lines of splitting, as seen 

 upon a horizontal surface {i.e. the sh-iJce) is a very well-known and 

 characteristic feature of cleavage ; in the neighbourhood of Aberyst- 

 wyth this direction is about N.N.E. to S.S.W., varying only a little 

 northwards or southwards. This being the constant direction (with 

 rare exceptions) of the exposed edges or strike, the slate may stand 

 vertical, or may slope W.N.W. or E.S.E. ; most commonly the dip is 

 about 80° W.N.W. When the planes of bedding are at all nearly in 

 the same direction as the cleavage, the latter will accommodate itself 

 to the lines of bedding, and run along them. Such a change in the 

 cleavage is very prettily seen in a road-side cutting between 

 Glandovey and Machynlleth. 



In practice, however, much caution is required in registering the 

 dips of cleavage planes, because of the phenomenon known as 

 " surface deflection." Very generally we find on the sides of the 

 hills that the slates have more or less toppled over in the down- 

 hill direction, so that a surface slate which should be vertical 

 now inclines inwards and downwards towards the centre of the 

 mountains. The numerous small openings for slate and lead, and 

 the deep water-gulleys enable us to find the true direction. 



This phenomenon is b}'^ no means confined to slates, but is also 

 well seen in the thick beds of limestone and sandstone, in basaltic 

 columns, etc. To explain its origin it is quite unnecessary to appeal 

 to the force of glaciers and the grounding of icebergs, for it is 

 obvious that, given such loosening agents as the roots of plants, and 

 especially the alternate freezings and thawings during winter, 



^ The rock structure we have been describing above is known as cleavage, and a 

 slate is the result, fihale is a rock which splits into thin layers along planes pro- 

 duced by the circumstances of its deposition. As a rule, slate is harder and more 

 regular than shale, and especially slates often split across the planes of bedding ; 

 shale rarely does so. 



