48
BATHYMETRICAL SURVEY OF
Loch Lubnaig originally extended to a point below Coireachrombie,
about three-quarters of a mile below its present outlet. This point has
been silted up by the detritus laid down by the Stank and Anie burns.
The original southern termination of the lake touched the rocky barrier
formed by the Leny grit. It is worthy of note also, that the level of
Loch Lubnaig has been lowered about 20 feet by the denuding action of
the river Leny.
Loch Katrine.—For a distance of 4 miles, west from Brenachoil Lodge
to Stronachlachar—about the half of the total length of the loch—this
lake has a comparatively flat bottom, enclosed by the 400-feet contourline. The deepest sounding in Loch Katrine, 495 feet, is at the eastern
limit of this basin, nearly due south of Brenachoil. The chart shows
that the soundings throughout this basin gradually increase in depth
eastwards to Brenachoil Lodge. The position of the deepest sounding
is of interest, seeing that the strata which form the floor of the lake at
this point consist of schistose micaceous grits, to the north-west of the
epidotic grits ('' Green Beds '') and the Ben Ledi grits, the two latter
groups having formed the great rocky barrier at and above the outlet of
the lake.
Near the upper end of the loch a rocky barrier crosses the lake from
Portnellan by the Black island to Rudha Maoil Mhir an-t Salainn. The
deepest sounding along this barrier is 90 feet, and the shallowest is 48
feet. On its lower side the 100-feet contour-line well-nigh crosses the
lake. Above it there is another basin over half a mile in length, the
greatest depth of which is 128 feet, immediately in front of the rocky
ridge just referred to. Westwards the lake shallows, and at its head
it has been silted up for a distance of half a mile by the alluvium laid
down by the Gyle river.
Below Brenachoil Lodge the soundings show an uneven floor, due
probably to ridges of rock rather than to morainic deposits, if we may
judge from the geological features on both sides of the lake. Ellen's
isle is composed of epidotic grits (" Green Beds "), and the promontories of Am Priosan partly of " Green Beds " and partly of Ben Ledi
grits. The promontory between the pier and the sluice is formed of
Ben Ledi grits.
During the geological survey of that region several small faults were
found to cross Loch Katrine, but these are of minor importance, and
have produced locally a slight brecciation of the strata. It is a typical
example of a rock basin. The deepest sounding occurs in the front of
the great rocky barrier in the lower part of the lake, in accordance with
what we might naturally expect on the theory of glacial erosion.
Though the soundings prove the deepest part of the lake to be 131 feet
below sea-level, yet this depth is in proportion to the vast thickness of
the ice during the successive glaciations of the basin.
Loch Achray.—This lake forms one basin, the deepest part being