1 84 PREHISTORIC E UR OPE. 



distance ; others have not travelled so far ; while frequently we 

 may see blocks lying quite close to the parent mass from which 

 they have been wrenched. I have described the rocky pave- 

 ment over which the ice flowed as often showing a planed, 

 smoothed, and striated surface. In many cases, however, we 

 find, instead of all this planing and polishing, only a jumbled 

 accumulation of large blocks and broken debris under the till. 

 The bed-rock has been smashed and crushed, and large masses 

 have been pushed out of place, the boulder-clay often appear- 

 ing tightly rammed between the blocks. This frequently char- 

 acterises much-jointed rocks, like certain sandstones and igneous 

 rocks. And one can see that the dislocated fragments have 

 been dragged along in the same direction as that followed by 

 the trend of the glacial stria? and drums of till in the same 

 neighbourhood. Here we observe the beginning of the process of 

 boulder-clay-making. As we follow the fragments of the same 

 disrupted rock which occur in the till farther down the valley, we 

 note how they become smaller in size ; while the sharp corners 

 at the same time get rubbed away, and the surfaces assume the 

 characteristic glacial markings. The stones and boulders in 

 the till thus vary much in size — from mere grit and small frag- 

 ments no larger than a hazel-nut up to great blocks measuring 

 many feet and even yards across. These last, however, are the 

 exception, and are generally met with at no distance from their 

 parent stratum. Large, far-travelled boulders in the till are 

 always well abraded, and invariably consist of some hard, 

 durable rock. Considerable lumps of soft sandstone and friable 

 shale, on the other hand, have never been able to stand a long 

 journey under the ice. They rapidly broke up into small 

 pieces, and were ground and rubbed down into sand and clay. 

 The enormous pressure exerted by the ice is well shown in 

 these and other phenomena — more especially in the appearance 

 which the till not infrequently presents of having been forcibly 

 intruded into the strata over which it was dragged and rolled 

 by the superincumbent ice. Veins and tongues appear squeezed 

 between the interstices of the rocks, and sometimes sheets of 



