384 ME. A. STEAHAIf ON THE GLACIATIOI^ OF 



Railway- cutting at Spring's Branch, near Wigan) it consisted of a 

 great number of alternating bands of red claj^ and fine sand, very 

 regularly interstratified : in another (a brick-pit near Wigan) it was 

 finely and evenly laminated. These, it is true, are exceptional forms 

 of the Lancashire Boulder-clay, and I attach more importance to the 

 fact of its occurring on a large scale in horizontal sheets alternating 

 with sand and gravel as a proof of its being a stratified deposit. 



One of the most noticeable features also in the drifts of the 

 region under description is the great development of sand and gravel 

 in which all evidence of ice- action is entirely wanting. The valley 

 of the Weaver and of the Alyn and the Vale of Clwyd give examples 

 in Whales, while in the neighbouring parts of England are found the 

 great sand-banks of Ellesmere, Delamere, and other parts of Cheshire. 

 Such extensive deposits of thoroughly washed and sorted sand and 

 gTavel are not found in the mountainous parts of Wales and the Lake 

 District, where, on the other hand, the proofs of the former existence 

 of an ice-sheet are more convincing. That they are part of the 

 Glacial beds is clearly proved by the fact that in almost every case 

 they may be seen to be overlain by true Boulder-clay * ; and that 

 they are truly marine deposits has, I believe, never been questioned t- 



The question, however, on which disagreement exists is whether 

 the striae were produced before or during the submergence which 

 led to the deposition of these marine beds. 



The occurrence of a tough blue basement-clay packed with stones, 

 most of which are scratched, underneath the undoubted marine 

 drift of the Welsh coast has been previously noted. This " Till " J 

 has been attributed to the action of an ice-sheet passing northwards 

 and eastwards from the highest ground of North. Wales in a period 

 preceding the submergence, and the striae have been referred to the 

 same agent. 



The opinion of Mr. Mackintosh on this point has been already 

 quoted (p. 382). My own observations also tend entirely to the 

 conclusion that the striae are connected with the marine drifts, and 

 were formed during the submergence. 



In almost every case it is the uppermost and newest member of 

 the drift under which the striated rock-surfaces are found. Erom 



* ' Geology of the IsTeighbourhbod of Chester ' (Geol. Survey Memoir). 



t Sands and gravels with marine shells of recent species occur on Moel Try- 

 faen at a height of 1350 feet above the sea, on Minera Mountain at 1230 feet 

 (Mackintosh, Quart. Journ. Geol. Soc. vol. xxxvii. 1881), and on Halkin Moun- 

 tain at 956 feet. The total amount of submergence is estimated by Mr. Mack- 

 intosh to have been 1900 feet in North Wales (Proc. Chester Soc. Nat. Sci. 

 part 3, 1885). 



\ The word " Till " has been usually applied to an uustratified clay in which 

 boulders are very abundant, the name Boulder-clay being usually given to a 

 less stony and more purely clayey deposit. A Boulder-clay, however, shades 

 into a Till on approaching the source of the materials. The Chalky Boulder- 

 clay of Lincolnshire, for example, becomes locallj^ a chalk-till on the south side 

 of the Lincolnshire Wolds. (Geol. Survey Memoir on sheet 83, in the press.) 

 The relation between the Scotch Till and the Boulder-clay of the Lancashire 

 plains has been discussed by Mr. Mellard Eeade, Proc. Liverpool Geol. Soc. 

 Session 21 (1879-80), and Trans. Geol. Soc. Glasgow, 1880. 



