former existence of Glaciers in Forfarshire. 583 



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

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

 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 suffi- 

 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. Lyell is, that the unstratified boulder- clay becomes more and 

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

 consequence of the influx of distinct materials, but in the author's 

 opinion of the grinding down by the ice of the mud and other 

 detritus. 



Mr. Lyell then describes the phsenomena of the second district, 

 or Strathmore. Though this district may be considered as one 

 great strath, yet it is divided into many longitudinal ridges and 

 valleys. The former, sometimes 300 feet in height, are for the 

 greater part parallel to the strike of the old red sandstone, and are 

 generally covered to the depth of sixty or more feet with till and 

 erratics, derived from the Grampians and the subjacent strata. This 

 covering is so general, that the structure of the district can be de- 

 tected only in the ravines through which the principal rivers pass. 

 The till constitutes invariably the oldest part of the detritus. The 

 boulders which it contains sometimes exceed three feet in diameter : 

 on the north muir of Kerriemuir is a block of trap-rock, six feet by 

 five feet, and near it is a mass of mica-schist, nine feet long by four 

 feet wide and three high. The till has been ascertained by Mr. 

 Blackadder to fill, in many places, deep hollows in the sandstone, 

 which would become lakes or peat-mosses if the till were extracted. 

 This distribution of the detritus, Mr. Lyell observes, may be ex- 

 plained on the supposition that, if the cold period came on slowly, 

 the advance of the glaciers would push forward the detritus accumu- 

 lated at their termination, and fill up, wholly or in part, the lakes or 

 other cavities which they would encounter in their progress. Along 

 most of the river courses, and in the lowest depressions of Strath- 

 more, the till is covered by stratified sand and gravel. 



One of the most remarkable peculiarities of the transported ma- 

 terials of Forfarshire and Perthshire is a continuous stream, from 

 three to three and a half miles wide, of boulders and pebbles, trace- 

 able from near Dunkeld, by Coupar, to the soutii of Blairgowrie, 

 then through the lowest part of Strathmore, and afterwards in a 

 straight line through the lowest depression of the Sidlaw Hills from 

 Forfar to Lunan Bay, a distance of thirty-four miles. No great river 

 follows this course, but it is marked everywhere by lakes or ponds, 

 which afford shell-marl, swamps, and peat-mosses, commonly sur- 



