MR MILNE ON THE PARALLEL ROADS OF LOCHABER. 415 



Loch Tulla, about 3 miles in length, and 1 in breadth. A stream enters from its 

 east and west ends. Its surplus waters are discharged from its south side, by the 

 river Urchay. 



Two years ago, I discovered all round this lake indications of three levels at 

 which its waters had stood, the lowest being about 183^ feet, the second 277 feet, 

 and the highest 474 feet, above their present level.* Loch Tulla I roughly esti- 

 mated at 540 feet above the sea. This lake, therefore, extending originally to 

 about 6 miles in length and half a mile in breadth, had sunk 197 feet. — at which 

 level it had stood long enough to form the second shelf; it next sunk 93^ feet, 

 — when the third shelf was formed ; after which it sunk 18.31 feet, — viz., to the 

 present level of the lake. 



It is unnecessary for me to enter into the proofs, that what I am now describ- 

 ing are reaUy beach lines. Their perfect horizontality, which I ascertained by a 

 spirit-level, looking at them from 12 or 15 different places along the banks of the 

 lake, — their general conformity in sweeping round headlands, and retiring into 

 valleys or burn-courses, — and the extent of flat surface at the levels of the different 

 shelves, afford convincing and irrefragable proofs. 



The difficvdty here, as in other similar cases, is to discover, what could have 

 dammed up the lake so much above its present level. The blockage, whatever 

 it was, must have existed somewhere in the valley, through which the river Urchay 

 flows. The country, on all other sides of Loch Tulla, rises much higher than 500 

 feet above its present level. The two lowest shelves are traceable for some dis- 

 tance down the valley of the Urchay, — the middle shelf for about half a mile, and 

 the lowest considerably farther. My notion is, that this valley had been formerly 

 filled with a great accumulation of gravel and diluvial debris, which was gradually 

 eat away and lowered by the stream which issued from the loch. Accordingly, 

 there exist still, at and near Urchay Bridge, great heaps of unstratified gravel, 

 which clearly present only a remnant of what must have formerly existed. The 

 valley at this place, is a quarter of a mile wide ; and its sides rise far above the 

 required level. 



(2.) In the valley, at the head of which Tyndrum is situated, there are very 

 manifest indications of the beaches of an ancient lake, although the valley is now 

 occupied by only an insignificant stream. At StrathfiUan church, the lowest 

 terrace is about 50 or 60 feet above the stream, and may be traced continuously 

 for at least a mile down the valley. The stream has cut through this old lake 

 bottom, exhibiting beds of gravel, sand, and clay, which have been deposited and 

 arranged by the water. About 237 feet above this flat, there are, on the sides of 

 the hills on both sides of the valley, traces of a horizontal shelf, which can be 

 distinctly followed with the spirit-level from above Tyndrum village, down the 

 vaUey by Auchreach farm-houses, Enich farm-houses, and as far as Crianlarich 



* These measurements were made by a mountain barometer, checked by the sympiesometer. 

 VOL, XVI. PART III. 6 N 



