Geological Society. 515 



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

 towards the centre, and tlie 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 

 waters 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 barrier 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 barrier 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 effect, 

 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 

 previously deposited. The phaenomena exhibited by the till in this 

 district, Mr. Lyell conceives, might be well accounted for by supposing 

 the union of three or four large glaciers ; but he considers it difiicult 

 to explain the accumulation of the overlying stratified materials, the 

 top of which must be 600 feet above the level of the sea, and facing 

 the Strath. In following out the narrow ridge which intervenes 

 between the Proson and the Carity, during last October, in company 

 with Dr. Buckland, the latter drew the author's attention to a spot 

 half a mile south-west of the House of Pearsie, where the surface of 

 a porphyry rock was polished, furrowed and scratched. The quar- 

 rymen of Forfarshire also state as a general fact, that rocks of sufii- 

 cient hardness, when first laid bare, are smooth, polished and scored ; 

 and Mr. Blackadder has found on the Sidlaw Hills large boulders 

 of sandstone grooved and polished. Another general fact mentioned 

 by Mr. liyell is, that the unstratified boulder-clay becomes more and 

 more impervious in the lower part of the Grampian glens, not in 



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

 2 L 2 



