i;o VIGNETTES FROM NATURE. 



ancient glacier, the ice in this part must have 

 been crowded together into a narrow com- 

 pass, and thus squeezed hard against the 

 sides and bottom of the gorge by the pressure 

 of the great ice-sheet in the rear. If you 

 look at the rock anywhere around the lake 

 you will see that it is worn quite smooth and 

 deeply scratched with ice-marks like those 

 which occur just below the summer level of 

 a glacier in Switzerland at the present day. 

 So the rock-basin in which the tarn lies must 

 itself be a product of the scooping action of 

 the glacier. When the ice melted away 

 under the genial climate of the post-glacial 

 period, a little stream took the place of the 

 vast frozen mass, and this stream expanded 

 in the hollow till it filled the small lake and 

 then ran out at the lower end. Hence the 

 arrival of the trout in Llyn Gwernant must 

 necessarily date from some period not earlier 

 than the end of the last ice age. Whatever 

 peculiarities they may display when compared 

 with the parent type must have been de- 



