138 
BATHYMETRICAL SURVEY OF 
Surface ... 
50 feet ... 
100 „ ... 
42°-8 Fahr. 
42°-0 „ 
41°-7 „ 
41°-0 „ 
Dr. L. W. Collet, who took part in the survey, supplies the following 
notes on the formation of St. Mary’s Loch and the Loch of the Lowes : — 
“During the great Ice Age the Yarrow valley was occupied by a glacier, 
as is shown by the U-shaped section and the moraine matter on the slopes 
of the hills. In this valley we find the two picturesque lochs, St. Mary’s 
Loch and the Loch of the Lowes, which are both due to burn deltas dam- 
ming the valley. Three dams are manifest : (1) one situated at the very 
head of the Loch of the Lowes, formed by the junction of the deltas of two 
lateral streams, the Chapelhope burn and the Eiskinhope burn ; a small 
loch was very likely formed at one time behind this dam, as shown now by 
an alluvial tract, which is the result of the filling up of the loch by detrital 
matter brought down by the principal burn ; (2) one at the lower end of 
St. Mary’s Loch, due also to the junction of the deltas of two burns, the 
Kirkstead burn and the Thorny clench ; this dam held back at one time 
only one big loch ; (3) one formed by the deltas of the Ox clench and the 
Thirlestane burn, which divided the big loch into two separate ones, now 
represented by the Loch of the Lowes and St. Mary’s Loch ; this dam has 
led to the gradual filling up of the Loch of the Lowes by the detrital 
matter brought down by the streams, as shown by the soundings, the 
deepest recorded being 58 feet, while on the other hand the deepest sound- 
ing recorded in the upper basin of St. Mary’s Loch is 112 feet. 
“ St. Mary’s Loch is fed by many streams : the' first one on the western 
shore is the Summerhope burn, the detrital matter brought down by which 
causes a sinuosity in the 50-feet contour-line in that region ; the Mare 
clench has given rise to a small delta, which has little effect on the contour- 
lines; the most important tributary is the Megget water, which has laid 
down a huge delta protruding across the lake, forming a sub-lacustrine 
barrier, on which the maximum depth recorded is 88 feet ; the Copper 
clench has also formed a small delta, and has raised the floor of the bay 
into which it flows. 
“From a bathymetrical point of view, St. Mary’s Loch is divided into 
two basins separated by the sub-lacustrine ridge due to the deposition of 
material brought into the lake by the Megget water. The deeper basin is 
situated in the north-eastern part of the loch, and might be ascribed to the 
combined effect of the two glaciers, the Yarrow glacier and the Megget 
glacier, uniting at that place. It is difficult to decide whether St. Mary’s 
Loch is simply a barrier basin, or whether it partakes of the character both 
of a barrier basin and a rock basin.” 
Mr. James Murray supplies the following notes on the biology of St. 
Mary’s Loch : — 
“ During the discussion as to the suitability of St. Mary’s Loch as a 
