42 
BATHYMETRICAL SURVEY OF 
In the case of Loch Katrine, which is the largest and deepest of the 
lochs under consideration, there is a great rocky barrier at its outlet due 
to the Ben Ledi grits. Here they form a belt over a mile in breadth, 
and give rise to the rugged scenery so characteristic of that region. 
They appear on the crags of the Trossachs at the mouth of the loch, on 
the crest and slopes of Ben Venue (2393 feet), on Ben Bhreac (2295 feet), 
and on the heights round Ben An (1326 feet). The strike of these hard 
and durable strata is E.N.E. and W.S.W. — that is, at right angles to 
the outlet of the loch, and the beds are vertical or highly inclined. 
The potency of the Ben Ledi grits as a rocky barrier must have been 
considerably increased by the development of epidotic grits or Green 
Beds lying immediately to the north. The latter, though not so 
massive as the Ben Ledi grits, are hard and durable ; they are repeated 
by a series of compound folds for nearly a mile across the strike, their 
northern limit being near Brenachoil Lodge. Their trend is likewise 
north-east and south-west, and the beds are vertical or highly inclined. 
On both sides of Loch Katrine above Brenachoil Lodge the geological 
structure is widely different, for in this area the Ben Ledi grits, grey- 
wackes, and slates reappear in a highly schistose form, the strata 
dipping generally at low angles to the south-east. Over much of this 
region, as already indicated, the altered sediments merge into mica- 
schists owing to the development of mica. It is obvious that these 
materials would yield more readily to the agents of denudation than 
the massive pebbly grits of Ben Venue and the Trossachs. 
Loch Achray, which lies about a mile to the east of the outlet of 
Loch Katrine, is only about 88 feet below the level of the latter loch. 
A powerful fault or dislocation, trending north-east and south-west, 
crosses the head of the loch near the Trossachs Hotel, which brings the 
massive Ben Ledi grits to the west in contact with slates to the east. 
It is a true rock basin which has been excavated mainly in the group 
of less durable slates. 
Loch Vennachar is crossed by the great boundary fault, already 
referred to, along the Highland border, the floor of the eastern portion 
being composed of Old Bed Sandstone conglomerate, while that of the 
western part is formed of grits and slates belonging to the crystalline 
schists. Though there is a covering of drift on both sides of the lower 
part of the loch, still this sheet of water forms a true rock basin, for the 
Old Bed conglomerate is exposed in the river about 1200 yards below 
the outlet. 
Loch Drunkie presents several interesting geological features. On 
referring to the map it will be seen that an arm of this loch runs nearly 
east and west for upwards of half a mile ; the northern margin is com- 
posed of massive grits, while the southern margin and probably the floor 
of this branch of the loch is formed of less durable slates. Another 
arm of this lake runs N.N.E., in the direction of Loch Vennachar, the 
