120 THE BOOK OF NATURE STUDY 



The best available evidence of upward movement of land 

 will be given by such raised beaches as occur along the Durham 

 coast or that of Devonshire (Fig. 27). Deposits of beach pebbles 

 and sand, generally backed by what were evidently cliffs once cut 

 by the waves, all now at a considerable height above high tide, 

 may be compared with modern sea-beaches also backed by cliffs 

 which are reached by the waves of to-day. On the other hand, 

 old forests which must have grown clear of the water are to be 

 seen on the Lancashire and Cheshire coasts, in Cornwall, the Bristol 

 Channel and elsewhere, which are now covered by sea-water at 

 high tides. The latter furnish evidence of subsidence beneath the 

 sea. Other direct evidence is not easy to obtain, but enough 

 may be obtained from direct observation, or the study of maps 

 and photographs, to ensure the acceptance of the general fact. 



This once admitted, it will be seen that the problems of the 

 stratified rocks require, and give confirmatory evidence of, such 

 movement in past time. This is given by the succession of strata of 

 different composition formed under varying circumstances. Lime- 

 stone full of marine organisms and deposited in clear deep water 

 will be found resting on or covered by deposits of consolidated mud, 

 in the form of clay or shale, and these again by beds of sandstone 

 or pebble beds clearly laid down in shallower water, sometimes 

 under the influence of shifting marine currents. Seas must have 

 existed and must have become shallower or deeper with lapse of 

 time. Coal seams, relics of former forest vegetation, are inter- 

 bedded with sandstones and shales, and such a succession of coal 

 seams alternating with shallow-water beds indicates land conditions 

 alternating with periods of submergence under water. 



Strata are often found to be inclined or to dip in various 

 directions. These must have been originally deposited on flat 

 sea-beds, and their inclination clearly indicates upward or down- 

 ward movement after they were laid down. The inclination 

 seen is part of larger structures ; any dipping rock bed, if traced 

 far enough, being found to' rise and fall in waves or folds. The 

 crests of the waves are known as arches or anticlines (up-folds) 

 and the troughs as synclines (down-folds) (Fig. 28). Such struc- 

 tures are seldom to be seen in completeness, and their existence 

 has generally to be inferred from evidence pieced together from a 



