BOULDER-CLAYS. 



§ L VARIETIES OF BOULDER-CLAY. 



I. — One bed is so distinct in character and so easily recognised that one of the 

 writers of these pages proposed, some years ago, that to it the term " Boulder Clay " 

 should be entirely restricted,^ and this course will be followed in the present Monograph. 



This Boulder Clay' contains a large number of striated and polished stones which 

 have been worn down, and not broken, by the process through which they have passed. 

 These polished and striated surfaces are so freshly preserved that the stones could not 

 have been rolled on a beach subsequently to their production. Any trituration would 

 at once destroy the fineness of the glaciated surfaces. 



The included stones are chiefly traceable to the heights nearest the locality in which 

 the special bed under examination is found, although a distinct proportion have been 

 derived from the more distant mountains. In some cases the included boulders have 

 travelled from distances in the direction along which a glacier would naturally have 

 moved, according to the general conformation of the country. 



The colour, as well as the general character of the enclosed stones, is determined by 

 the mineral character of the district in which it is found. 



There is no distinct stratification, although there are occasional seams of sand and 

 clay. The Boulder Clay varies in tenacity ; but, as a rule, is toughly compact and hard 

 to be worked even with a pick-axe. 



Its thickness is excessively variable, extending from a mere surface-covering of the 

 rock to a depth of one or two hundred feet. 



The highest point at which the stratified clay containing glacial shells has yet 

 been found in Scotland is at Chappel Hall, near Airdrie, 526 feet above the sea; but an 

 unstratified and unfossihferous Boulder Clay, of the character just described, may be 

 traced to very considerable heights. It has been seen by Mr. J. Geikie on the tops of 

 the Ochils, 1500 feet; by Mr. Croll on the Pentland Hills, 1617 feet; by Mr. Milne 

 Home, near Schehallion, 2000 feet above the sea ; and it may be found in almost every 

 nook and corner of the Highlands. At lower levels it is very largely developed, both in 

 the plains themselves and on the flanks of the hills bordering wide valleys. 



While this Boulder Clay reaches to a height of at least 1500 feet beyond that at 

 which any arctic shell-clay has yet been discovered, it at the same time underlies 

 the shell-bed throughout the whole of the west of Scotland, as well as in some of the 

 Eastern districts. Many instances of this fact will be noted in the detailed descriptions 



1 See a paper "On the Tellina calcarea Bed at Chappel Hall, near Airdrie," by Henry \V. Crosskey, 

 in the 'Quarterly Journal Geolog. Soc.,' 18G5, vol. xxi, p. 219. 



