CHAP. XIII. FORFARSHIRE ZONE OF BOULDER CLAY. 249 



consider to belong to the same period; namely, a continuous 

 zone of boulder clay, forming ridges and mounds from fifty 

 to seventy feet high (the upper part of the mounds usually 

 stratified), enclosing numerous lakes, some of them 

 several miles long, and many ponds and swamps filled 

 with shell-marl and peat. This band of till, with Grampian 

 boulders and associated river-gravel, may be traced con 

 tinuously for a distance of thirty-four miles, with a width of 

 three and a half miles, from near Dunkeld, by Coupar, to the 

 south of Blairgowrie, then through the lowest part of Strath- 

 more, and afterwards in a straight line through the greatest 

 depression in the Sidlaw Hills, from Forfar to Lunan Bay. 



Although no great river now takes its course through this 

 line df ancient lakes, moraines, and river gravel, yet it evi 

 dently marks an ancient line by which, first, a great glacier 

 descended from the mountains to the sea, and by which, 

 secondly, at a later period, the principal water drainage of this 

 country was effected. The subsequent modification in geo 

 graphy is comparable in amount to that which has taken 

 place since the higher level gravels of the Valley of the 

 Somme were formed, or since the Belgian caves were filled 

 with mud and bone-breccia. 



Mr. Jamieson has remarked, in reference to this and some 

 other extinct river-channels of corresponding date, that we 

 have the means of ascertaining the direction in which the 

 waters flowed by observing the arrangement of the oval and 

 flattish pebbles in their deserted channels ; for in the bed of a 

 fast-flowing river such pebbles are seen to dip towards the 

 current, as represented in fig. 35, such being the position of 

 greatest resistance to the stream.* If this be admitted, it 

 follows that the higher or mountainous country bore the 

 same relation to the lower lands, at the time when a great 

 river passed through this chain of lakes, as it does at present. 



* Jamieson, Quarterly Geological Journal, vol. xvi. p. 349. 



