BRITISH POSTGLACIAL &> RECENT DEPOSITS 397 



Kinnoul Hill and that neighbourhood. 1 However that may be, 

 it is evident that the ice which lifted and floated off a boulder 

 measuring 4 feet in diameter must have been of considerable 

 thickness. Nevertheless, the mere occurrence of those few 

 sporadic erratics can hardly be appealed to as conclusive evi- 

 dence of colder climatic conditions than now obtain. During 

 the winter of 1878-79 I saw ground-ice forming rapidly on the 

 bed of the Tay at Perth ; and shortly afterwards the river, which 

 is tidal at that place, was frozen over to a depth of 12 and 

 14 inches. When the ice broke up and floated away, it must 

 have carried seaward a goodly number of small erratics, for I 

 noticed gravel -stones in several blocks of ice which were 

 stranded on the banks opposite Kinfauns. It is rather the 

 general character of the Carse-deposits themselves than the pre- 

 sence in these of a few sporadic erratics which appears to indi- 

 cate colder climatic conditions. The fine, tenacious brick-clays, 

 and even the less cohesive silty or loamy clays, cannot be 

 likened to the dark sludge and slimy silt and mud which are 

 now gathering upon the estuarine bed of the Tay, but they 

 closely resemble, and even in many cases are identical in 

 character with, those laminated clays of true glacial age which 

 contain Arctic shells. The rivers which flowed into the ancient 

 estuary of the Tay at the time the Carse-beds were forming 

 appear to have been abundantly charged with finely-levigated 

 matter. They must have been, in short, extremely muddy 

 rivers ; and some idea of the quantity of material they carried 

 in suspension may be gathered from the fact that the Carse- 

 clays cover an area of not less than 35 square miles, to a 

 depth varying from 10 to 40 feet and more. And to this we 

 must add also the material which was swept out to the open 

 sea, as well as the great loss by denudation which the clays 

 have experienced since the sea finally retreated to its present 

 level. 



1 Porphyrites occur also along the foot of the Grampians, near Dunkeld ; and 

 it is just possible that the boulder in question may have been drifted by ice down 

 the ancient Tay from that neighbourhood. 



