522 THE DRIFT OR GLACIAL FORMATION OF 



retained. True, some of the shells are scratched, and in such cases must have 

 been dragged a little. But how the valves have been so generally disjoined 

 it is not easy to explain. 



That the Boulder-clays are marine deposits does not, I think, admit of 

 doubt ; they are frequently stratified, they often contain passage-beds 

 between them and their associated sand and gravel beds, and they shade 

 into laminated clays interbedded with them. These, taken with the fact 

 that they often contain an abundance of marine fossil remains, afford strong 

 proofs that Boulder-clays have been deposited in the sea. 



The coast of Ayrshire lies well open to the Atlantic, and it is difficult to 

 see how its drift could be a deposit from land-ice carrying " ground-moraine " 

 uphill, as some geologists maintain. It is generally admitted that before the 

 glacial period the land stood much higher than it does at present, but this 

 only increases the difficulty of understanding how marine remains could 

 have been transported uphill by land-ice. 1 



But how are the drift-shells so uniformly single-valved ? It may be 

 suggested that after the large ligaments have decayed or been eaten by 

 microzoa the valves would easily fall asunder by agitation of the water 

 caused by the falling of stones from floating ice, and probably a large pro- 

 portion of the shells were smashed by the same agency. 



DRIFT PECULIARITIES. 



The Original Ground- Moraine is a thin layer on top of the rock which 

 differs from any of the drift above it, and is, I believe, the only deposit of 

 the kind ever formed by land-ice. Where the drift has been dragged (a) by 

 the stranding of icebergs, (6) by glaciers pushing into the sea amongst 

 deposits, or (c) by subsequent 'land-ice dragging the beds, it has been con- 

 verted into what may be called secondary ground-moraine. 



Boulder-clay is seen sometimes resting on rock entirely different in colour 

 from itself even at the point of contact, and it is often difficult to know, on 

 the one hand, where the Boulder-clay ends and the gravel or laminated clay 

 begins. On the other hand, it is puzzling to decide as to whether a bed is 

 simply clay or Boulder-clay, as sand, gravel, and clay often contain large 

 blocks, and gravel is sometimes very clayey, as may at present be seen at 

 Blackhall, near Paisley, where it also contains boulders, is stratified, and lies 

 under true Boulder-clay. 



Unscratched Angular Stones, well-scratched stones, and unworn and un- 

 scratched shells, are often got lying near each other in Boulder-clay. 



Boulders versus Stones. The latter are found often in the drift, whereas 

 boulders are the exception, though they are conspicuous from their size. 



Lines of Boulders are frequently seen in sections in the Boulder-clay, but 

 the arrangement is only noticeable when they extend over a considerable 

 space. 



Ripple-marked Sand is occasionally seen interbedded in the drift, as in 

 sections on the Guelt Water. 



Manganese Gravel. Black gravelly beds stained by the binoxide of man- 

 ganese are sometimes seen in the drift. 



Drift on Steep If ill-sides seen in sections on the burns is always very stoney 

 and sometimes stratified. 



Crumpled Mud is occasionally met with in the drift under normal beds. 

 Vertical Burrows in Sand are found in thick Boulder-clay at Newmilns. 2 



Sand-dykes in Boulder Clay are rare, but in. Crawfordland Burn eight are 



1 The Norwegian expedition to Greenland of 1876, 1878 found remains of a littoral 

 fauna down to 6000 feet below sea-level. 



2 Trans. Geol. Soc, of Glasgow, vol. xi., supplement, 1898, p. 31, fig. 10. . 



