DRIFT OR GLACIAL FORMATION. 521 



At present it is the fashion to regard the close of the glacial period as of 

 comparatively recent date 8000 years even having been suggested. North of 

 Portincross there is a Raised Beach cliff a mile long, composed of reddish 

 sandstone and pink trap, rising to a vertical height of 300 feet, west of 

 which, during the Raised Beach period, a quantity of rock of probably at least 

 9,000,000 of tons must have been weathered from the face of the retiring 

 cliff, ground into sand and mud, and carried off by the waves and currents. 

 The talus of this cliff amounts to more than 100,000 tons. Where the rock 

 is composed of sandstone and conglomerate the talus is a reddish clayey sand 

 with pebbles and large masses of sandstone, but where the sandstone is capped 

 by trap, 150 feet thick, the talus has also large blocks and shivers of trap. 

 Some of the blocks from this cliff rest on Raised Beach shelly beds which 

 they have protected from decay. 



Advocates of the theory of a short period of time since the close of the 

 glacial period must bear in mind that there are no extinct shells in the 

 Eaised Beach beds of the Clyde, nor any boulders, and that little or no 

 alteration in sea-level has taken place since Roman times, and very little 

 since Iverian ones. 



Above sea-level, the drift rises to at least 1750 feet, as already stated, and 

 like that below it contains several beds of Boulder-clay sometimes four. 

 On some parts of the coast-line there are higlAnd old "scars " of drift, as N. of 

 Girvan and N. of Ballantrae. An exposure 3 miles N. of the latter shows : 



Boulder-clay, with a few shelly fragments, . . 98 feet. 

 Reddish sand, with decaying shells, . . \ 28 

 Dark-red clay, with a few stones and solid shells, J 

 Gravel and sand, with rotted shells, . . 21 



Boulder-clay, only occasionally seen, . . . 5 



The base of this section, not seen, is probably about sea-level. 



The drift-beds as we ascend in altitude generally become thinner, and at 

 about 1100 feet, in Ayrshire at least, no intercalated beds have been recorded 

 only Boulder-clay/ In a recent investigation of the deposits cut through 

 by the new railway from Elvanfoot to Wanlockhead in Lanarkshire I 

 observed that there is no Boulder-clay above 1000 feet, and that all the drift 

 is well-stratified, sometimes containing large foreign blocks. 



In no part of Scotland are the drift- deposits better seen, perhaps, than in 

 Ayrshire, the strata in many of the sections lying almost horizontal, or 

 dipping with the valleys. There is clear evidence of extensive deforma- 

 tion having taken place, sometimes during their deposition, but occasionally 

 after it, and, there is an almost continuous exposure of the shelly-drift from 

 sea-level to a height of 1061 feet above it. 1 The shells are _ generally 

 preserved in the Boulder-clay or in stoney clay, though sometimes in sand or 

 gravel, or in clay layers in the sandy beds and in laminated clay. 



In Lanarkshire shelly-drift occurs on the Douglas Water at 1060 feet, and 

 for two miles down stream. Recently I have obtained marine fossils at even 

 a higher level on the Powbrane Burn. In other parts of the Clyde area shells 

 have been found in the drift from near to over 1100 feet above sea-level. 



SHELLS IN THE BOULDER-CLAY. 



It must have been noticed by most observers that shells in the Bo alder- 

 clay exist almost always as fragments or as single valves, though these are 

 often in such a fine state of preservation that it is impossible to suppose 

 they have been transported any distance. The fine teeth of the most 

 delicate Leda are in the most perfect preservation, and the epidermis is often 



1 Trans. Geol. Soc. of Glasgow, vol. xi., supplement, 1898, p. 113. 



