582 Geological Society: — Mr. Lyell on the 



difficult of explanation. Seen from below, this barrier resembles an 

 artificial dam "200 feet high, with numerous hillocks on its summit. 

 On the eastern side it appears to have been denuded to the extent 

 of about 300 yards by the Esk. Its breadth from north to south 

 is about half a mile. The lower part, 30 feet in depth, laid open 

 in the river cliff, consists of impervious, unstratified mud, full of 

 boulders ; but the total vertical thickness of this deposit is stated to 

 be from 50 to 80 feet ; and the upper part of the barrier is com- 

 posed of from 50 to 100 feet of very fine stratified materials. It is 

 not possible, Mr. Lyell observes, to account for the accumulation of 

 this barrier by the agency of water, particularly as no tributary 

 joins the Esk at this point ; but if the barrier be supposed to be the 

 large terminal moraine of a receding glacier, then its form and 

 position, he says, are easily to be understood. M. Agassiz, in his 

 work on glaciers, shows, that when these masses of ice enter a nar- 

 row defile from a broader valley, the lateral moraines are forced 

 towards the centre, and the mass of transported matter is spread 

 more uniformly over the whole. Such a terminal moraine left by a 

 receding glacier in a defile, Mr. Lyell states, would dam back the 

 watei-s of the glacier, and produce a lake ; and the phaenomena pre- 

 sented by the barrier of Glenairn, and the plain which extends in 

 its rear, are fully explicable on the assumption of their having been 

 produced by a glacier. The stratification of the upper portion of 

 the bai-rier is also shown to be partly in accordance with the effects 

 produced by the formation of ponds of water on the surface of mo- 

 raines ; but Mr. Lyell states, that the accumulation of so great a 

 capping of stratified materials is still the most obscure character of 

 the deposits under consideration. 



At Cortachie, about four miles below the bamer of Glenairn, the 

 South Esk enters the country of old red sandstone, and a mile and 

 a half lower it is joined by the Proson, and a mile yet lower by the 

 Carity. In the district in which these streams unite there is a con- 

 siderable thickness of unstratified matter full of Grampian boulders, 

 and covered for the greater part with stratified gravel and sand. In 

 some cases the latter exhibit the diagonal laminae common in sub- 

 aqueous formations ; and in others the strata are so contorted, that 

 a perpendicular shaft might intersect the same beds three times. In 

 the latter instances the surface of the subjacent red boulder clay 

 has not partaken of the movement by which the stratified deposit 

 was contorted; and in consequence Mr. Lyell ascribed the eflFect, 

 when he first beheld it in 1839, to the lateral pressure of large 

 masses of drifted ice repeatedly stranding upon a shoal of soft ma- 

 terials*. In the middle of the tract between the South Esk and the 

 Proson is a dry valley, and to the south of this valley, near the Pro- 

 son, an excavation was made ten years ago, which exposed extremely 

 contorted beds overtopped by others perfectly horizontal, having 

 been formed by tranquil deposition after the disturbance of strata 

 pre^^ously deposited. The phsenomena exhibited by the till in this 



* See Proceedings, vol. iii. p. 178. 



