SAND DUNES. 125 



reaching the sea, and often owes its existence to a bed of clay 

 which had been exposed down to a low level on that coast, and 

 eaten back by the sea into a bay. This bay is usually a point from 

 which the adjacent harder rocks may be undermined, for drained 

 of the moisture they contained, owing to the dip of the strata, their 

 substance contracts and becomes divided by innumerable cracks and 

 division planes, separating into blocks which have no support or firm 

 coherence with the mass of the stratum, when the underlying portion 

 between tide-marks has been removed. After falling, these fragments, 

 when hurled back by the tidal waters, become battering-rams for 

 making further inroads into the sea-wall of rock, and thus the process 

 goes on, governed by the direction of the wind and the currents which 

 move the water. The height of a cliff is governed chiefly by the 

 height of the adjacent land. On some parts of the west coast 

 of Scotland, the height of cliffs is immense ; and, as a rule, among 

 the contorted and upheaved Primary formations cliffs are higher 

 than among the newer formations. But the waste is less rapid, 

 and the cliffs often show in their retreat from the shore, in their 

 upper portions, evidence of denudation, and different relative posi- 

 tions of land and water to those which exist now. The Secondary 

 rocks, from their loose texture, have wasted at a more rapid rate, and 

 the cliffs are often high, because easily undermined, and so eaten back 

 that the traces of earlier denudation have become obliterated. The Ter- 

 tiary cliffs of the east and south-east of England are mostly of moderate 

 height, because the level of these deposits rises so little out of the sea, 

 as may be seen in the Crag formation at Felixstow and Aldborough, 

 while on many parts of this coast of Suffolk cliffs have no existence. 



Sand Dunes. Though the sea thus destroys the coast, and 

 forms cliffs, yet the winds often protect the shores where no cliffs exist. 

 On sandy coasts, as the tide runs down, the sand left dry is caught up 

 by the wind and blown inland, only to be arrested by vegetation stop- 

 ping its movement at *a little distance from the water. A mound thus 

 formed becomes permeated and reticulated by the roots of those plants 

 which specially luxuriate in these conditions, and year by year the 

 mounds may grow in height until a ridge of hills, or sand dunes, 

 often of considerable height, is formed guarding the shore. In our own 

 country these sand hills are less developed than on flat continental 

 coasts ; but on some parts of Norfolk, South Wales, and Cornwall, 

 good examples of sea-coast scenery of this kind may be seen. Sir 

 Charles Lyell published a sketch of the church tower of Eccles in 

 Norfolk covered up in a sand dune, as it appeared in 1839 ; but the 

 winds which heap up the sand may reverse their direction, and in 

 1863, when we last visited this part of the British coast after unusually 

 strong equinoctial gales which scoured all the cliffs clean in the north 

 of Norfolk, every trace of the outer range of these sand dunes was 

 gone. The church remained clean and roofless by the shore, the fields 

 which had been newly ploughed before the sand had covered them 

 were laid bare, as fresh as though the furrows were newly turned, even to 

 showing the prints of the hoofs of asses which had drawn the small and 

 primitive plough. Prior to the formation of the sand hills the waste 



