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 numerdtis 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 continuously for a dis- 

 tance of thirty-four miles, Avith a width of three and a half 

 miles, from near Dunkeld, by Coupar, to the south of Blair- 

 gowrie, then through the lowest part of Strathmore, 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 of 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- 

 gTaphy 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. svi. p. 349. 



