LAKES IN RELATION TO GEOLOGICAL FEATURES 503 
widen near the upper end of the basin, probably because the rocks 
consist of alternations of strong and weaker schistose grits. This 
basin is continued in a north-and-south direction till it reaches an 
outcrop of strong grit forming Ross Point which juts far out into the 
loch from the eastern shore. The grit is obliquely truncated by a 
fault that brings the mica-schists against the Luss slates on the west 
side of the lake. Hence the rock-basin is continued round the 
Ross Point in the softer strata. Beyond this promontory the loch 
widens in the line of outcrop of the Luss slates. Below Luss^ the 
slates are succeeded by massive pebbly grits, forming a shallow 
plateau on which are situated the islands of Inchtavannoch and 
Inchlonaig. Below the lOO-ft. contour line only a narrow channel 
between Inchlonaig and Strathcashell is to be found crossing this 
rocky barrier, the others being much shallower. 
In the lower section of the loch the valley widens, and opposite 
the Endrick and the Fruin it merges into the Lowland plain. This 
change in the configuration of the surface may be attributed to the 
Highlands schists having been covered by strata of Old Red Sand- 
stone age. The sudden widening of the loch l)elow the islands of 
Inchtavannoch and Inchlonaig is evidently due to the removal in 
pre-glacial time of the Upper Old Red sandstones and conglomerates 
from the old plain of denudation of Highland schists upon which 
they were originally laid down. Between a line drawn across the 
lake from Rossdhu House to Arrochymore Point and the Highland 
Border fault, which traverses the islands of Inchcailloch and Inch- 
murrin, the floor of the loch is formed of Upper Old Red Sandstone 
strata, which there dip at comparatively low angles to the south-east. 
The shatter-belt of the Highland fault is here much indurated with 
carbonates, and is flanked on the south side by nearly vertical beds 
of Lower Old Red conglomerate. These together form a prominent 
barrier and a chain of islands. The conglomerates are succeeded by 
the softer sandstones dipping less steeply to the south-east. As 
might be expected from the widening of the valley and its coal- 
escence with the plain, this part of the lake is shallow. The lowest 
portion may not be a rock-basin, for the valley of the Leven is floored 
with raised beach deposits and alluvium. 
There is evidence to prove that in late glacial time the lower 
part of the loch was an arm of the sea, for deposits of clay with 
arctic shells are found at Rossdhu and on Inchlonaig, which are 
supposed to belong to the 100-ft. raised beach. This shelly clay 
has not been met with higher up, from which it may be inferred 
that the upper part of the lake was occupied by the retreating 
glacier during the time of its deposition. If this correlation be 
correct, there must have been a recrudescence of glacial conditions, 
for the Inchlonaig deposits are overlain by a red shelly boulder clay 
which can be traced far beyond the present foot of the lake. The 
