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 933 feet, 
—when the third shelf was formed ; after which it sunk 1834 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 really 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 difficulty 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 Strathfillan 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 
valley 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. SN 
