50 TV. Whitaker — Geology of Coasts of England and Wales. 



split up into large masses or even into small blocks. Faults, or planes 

 along which the rocks have been displaced, are sometimes effective in 

 this way; but far less so on the whole than the far more common 

 planes of division (but not of displacement) known as joints. These 

 are vertical or highly inclined to the bedding-planes (when the rocks 

 are of sedimentary origin) ; there is often one dominant set, running 

 in one general direction and therefore approximately parallel ; but 

 these master -joints are generally crossed by others at various angles. 



The whole arrangement, therefore, of these divisional planes, which 

 sometimes form narrow fissures, is one that favours the separation of 

 the rocks into blocks, which may be more or less readily displaced. 



In the case of sedimentary rocks the bedding-planes, that is the 

 planes of deposition, may greatly assist in the destruction of even the 

 hardest rocks. 



It is not, however, the nature and structure of a rock that alone 

 determine its rate of erosion. Its relation to other rocks is equally 

 important ; for instance, if a harder rock is overlain by a softer one, 

 the resultant form of cliff will be what is known as slope over wall; 

 the hard rock will resist the action of the sea at the foot and will 

 form a more or less vertical cliff, whilst the soft rock will yield readily 

 to the atmospheric agencies that are constantly in action above, parts of 

 it being washed over or slipping over the cliff beneath, until the angle 

 of rest is reached and a condition of comparative stability is gained. 

 This form of cliff then is of the stable kind, depending' on the 

 resistance of a firm rock to the sea. 



"Where a harder rock is underlain by a softer one we get the form 

 of wall over slope. The lower beds cannot stand with anything like 

 a vertical face, but weather down to the angle of rest, modified as 

 this may be by the softer bed having to support the harder one above. 

 This form, therefore, is of an unstable kind, as the sea can readily 

 wash away the yielding bed and so bring about the destruction of the 

 hard overlying mass, from want of support. This, however, is one of 

 those cases in which nature provides a check on her own destructive 

 power ; the blocks of hard rock that fall down the slope form a natural 

 breakwater, hindering the sea from attacking the softer rock until 

 they have been removed. There may, of course, be more than one 

 slope or wall, in the case of alternation of harder and softer rocks, and 

 the result will be a broken slope of considerable extent. 



Another kind of alternation, that of permeable and impermeable 

 rocks, is also important, as causing springs or general outflow of 

 water along definite lines, thus adding to the slippery nature of the 

 impermeable beds and facilitating the sliding down of the permeable 

 beds above, which are often in themselves firm and resisting. Our 

 great landslips are largely owing to this cause. 



The direction of the dip in sedimentary beds is also a powerful 

 factor in coast- erosion. Should that direction be in any way seaward, 

 of course it favours the seaward slipping of the beds, hard or soft; 

 but should it be landward it opposes an obstacle to such slipping, even 

 in yielding beds, and delays the loss of land. 



The kingdom of England and "Wales is noted for the great variety 

 of rocks that occurs on its coast and for the frequent changes from 



