SOUTH LANCASHIRE, CHESHIRE, AND THE WELSH BORDER. 385 



the tendency of the drift to be iu horizontal beds, filling up old valleys 

 and hollows, it results that the upper beds frequently overlap the 

 lower, and that towards the margin of a deeply drift-covered area, 

 the newest member, usually a Boulder-clay, is found resting directly 

 upon the rock. It is precisely in such situations that most of the 

 striae have been observed. But it is certain, from their freshness, 

 even upon the friable Triassic sandstones, that they must have been 

 covered immediately after they were produced, and must have 

 remained so up to the present time. It follows, then, that they 

 were formed after the earlier and before the latest of the marine 

 deposits. From the general similarity from top to bottom presented 

 by the whole series of the drift-beds in the plains *, it seems im- 

 probable that there can have been such complete breaks in the 

 sequence of events as the repeated encroachments of an ice-sheet 

 during the submergence of the area. There is less difficulty in 

 conceiving, on the other hand, that the sea was subject to periodical 

 invasions by floating ice, by the interruption of which the free 

 sorting and stratification of sediment would be checked, and a sheet 

 of Boulder-clay formed, while by the irregular drifting and occa- 

 sional stranding of the ice, striae, such as are observed, would be 

 produced. 



The evidence for the extension of a portion of the Lake-District 

 ice-sheet as far south as Xorth Lancashire has been given by 

 Mr. Groodchild t, and consists in the position and general direction 

 of the striae and in the distribution of the drift, which he considers 

 to have been carried in the ice. 



The contiguous district on the south had been previously described 

 by Mr. Tiddeman J:, who states, as a result of his observations, that 

 " the general movement was to the S. or S.S.E., across deep valleys 

 and over hills of considerable elevation." He finds that the rock- 

 surface is invariably moutonnee and usually striated where Till exists 

 or has existed ; that there are no indications of coasting or of deflection 

 by partially submerged hills ; that the terminal curvature is found 

 on the flanks as well as on the tops of the hills ; lastly, that the 

 transportation of the Till is explicable onlv on the supposition that 

 it was the moraine prof onde of an ice-sheet. And for these reasons 

 he attributes the observed effects to the action of an ice-sheet rather 

 than of floating ice. 



It will be seen, however, from what has been said above, that 

 there are great differences between the region described by Mr. Tid- 

 deman and that now under consideration, the principal being the 

 comparative rarity of moutonnees or striated surfaces, and of terminal 



* There is sometimes a larger proportion of local materials in the lower 

 beds, as might be expected. So great, however, is the resemblance of one part 

 of the series to anotlier that the name of Upper and Lower Boulder-clay has 

 been assigned to the same bed by different ol.iservers. It has not been found 

 possible to make any general separation of Upper and Lower Boulder-clay in 

 South Lancashire. 



t Quart. Journ. Geol. Soc. vol. xxxi. (187.^). 



