James Geikie — On Changes of Climate. 549 



between the ice and the rock, the tear and wear of the latter would 

 cease. But when we bear in mind the vast weight of the moving 

 mass of ice, we need surely have no difficulty in conceiving how 

 this ice-sheet must have acted upon the Boulder-clay much in the 

 same way as a river acts upon the detritus in its bed. For just 

 as a stream will sometimes pile up long banks of mud, sand, and 

 gravel, and after a time sweep these away again, so the old ice-sheet 

 must frequently have ploughed out the materials which gathered 

 below it, and squeezed and pushed them from one position to an- 

 other. There is plenty of evidence to show that the ice which moved 

 over the central districts of Scotland was ever and anon deflected — 

 now towards the south by the powerful current of ice that set in 

 from the Highlands, now to the north by the strong current that 

 swept outwards from the southern uplands. During such changes 

 in the course of the ice-flow, the underlying Boulder-clay would 

 come to be shifted about by the varying direction and pressure of 

 the ice. Again, there are wide districts in the Lowlands where 

 Boulder-clay either does not occur at all, or is represented by only 

 a few little patches ; nor is this always, or even often the result of 

 subsequent aqueous denudation. The deposition of the Till has 

 been much less continuous and wide-spread than is generally be- 

 lieved ; but the evidence on this head can hardly be given here. 



But even if it be admitted that Till not only was formed, but also 

 accumulated underneath the ice, being dragged and squeezed forward 

 from one part of the bed of the mer de glace to another, how, it may 

 be asked, can we account for the presence below and in the Boulder- 

 clay itself of gravel, sand, and clay ? How have the beds below the 

 Till escaped destruction, and in what manner did that Till come to 

 contain accumulations of water-assorted materials? The beds to 

 which I allude have long been known to geologists, but their occur- 

 rence has generally been looked upon as quite exceptional, and the 

 deposits themselves of too trifling an extent to merit much attention. 

 But a wider acquaintance with the superficial deposits has taught 

 us that the stratified beds of the Boulder-clay are neither so excep- 

 tional nor so insignificant. They occur not only very frequently, 

 but often attain a thickness of many feet, or even fathoms. Mere 

 lines of sand and gravel might perhaps be ignored or explained 

 away ; but when we come to deal with deposits upwards of thirty 

 or seventy feet and more in thickness, it is evident we can no longer 

 consider them in the light of accidental accompaniments of the 

 Boulder-clay in which they occur. Yet it needs only a very super- 

 ficial examination to assure one that the intercalated beds are 

 merely the wrecks of what they must at one time have been. Over 

 and over again we find them twisted, bent, crumpled, and confused, 

 often in the wildest manner. Layers of clay, sand, and gravel, 

 which must have been deposited in a nearly horizontal plane, are 

 puckered into folds, and sharply curved into vertical positions. I 

 have seen whole beds of sand and clay which had all the appearance 

 of having been pushed forward bodily for some distance, the bedding 

 assuming the most fantastic appearance. But the intercalated beds 



