''POSTGLACIAL FORMATIONS" OF SCOTLAND 671 



those of the Tay, the Forth, and the Clyde. Usually it assumes the 

 form of well-marked terraces of gravel, sand, clay, and silt; but 

 on the more open seacoasts it is not infrequently represented by 

 ledges or benches cut in the solid rock. Most of the shells, etc., 

 which it contains belong to still indigenous species. 



Obviously a considerable interval of time separated the formation 

 of these two raised beaches. Before the 45-50 foot beach began to 

 be formed, the characteristic arctic species of the older beach had 

 disappeared from our coasts. Further, there is evidence to show 

 that after the 100 foot beach had been Hfted out of the water, it 

 was for a lengthy period subjected to severe erosion — more especially 

 in our estuarine valleys. Moreover, it is quite clear that this erosion 

 was effected by rain and rivers when the land stood at a relatively 

 higher level than it does today, and at a date long prior to the forma- 

 tion of the 45-50 foot beach. In the valleys of the Tay and Earn, 

 for example, the accumulations of the 100 foot beach have been 

 extensively trenched and swept away from broad tracts, so that 

 they now form terraces, the bluffs of which overlook the later carse 

 deposits of the 45-50 foot beach. That the erosion referred to was 

 not the work of the sea in which these younger estuarine beds were 

 formed is proved by the simple fact that the latter do not rest directly 

 upon the denuded deposits of the 100 foot beach. On the contrary, 

 they are separated from these by a widespread sheet of peat, and 

 this is directly underlain by river silt, clay, sand, and gravel. 



It is clear, then, that the latest stage of the Glacial Period was 

 accompanied or followed by a change in the relative level of land and 

 sea. The sea retreated to a lower level than the present, while 

 rivers plowed their way down through the deposits of the 100 foot 

 beach, and in time formed broad alluvial flats which were overlooked 

 on either side by the bluffs and terraces of the denuded shelly clays. 

 By and by these younger " haugh-lands " were overspread with 

 dense vegetation — the general character of which betokens a climate 

 not less temperate than the present — the dominant species of trees 

 being oak, alder, hazel, birch, etc. This old land-surface is now 

 represented, as I have already mentioned, by a thick bed of peat — 

 the rootlets of trees and other plants penetrating the underlying 

 fluviatile deposits. 



