322 
BATHYMETRICAL SURVEY OF 
Surface 
2 feet 
3 „ 
3-5., 
4 ,, 
5 „ 
10 „ 
20 „ 
55° -8 Fahr. 
55° -7 „ 
55° -4 
55° -2 
53° -6 
53° -4 
53° 0 
53° T 
This series shows a range of 2°*8, there being a fall of no less than 
l°-6 between 3J and 4 feet. 
Loch an Dithreihh (see Plate LXXVI.). — Loch an Dithreibh (or 
Deerie, or Derry) lies less than 3 miles to the south of the head of the 
Kyle of Tongue, with Ben Loyal to the east and the lofty Ben Hope, a 
magnificent object in the landscape, to the west. The general trend 
of the loch is north-north-east and south-south-west, the main body 
of the loch trending almost north and south, and throwing out an arm 
towards the north-east. The loch is over miles in length, the main 
body being approximately uniform in width, with a maximum breadth 
of two-thirds of a mile, while the north-eastern arm is much narrower ; 
the mean breadth of the entire loch is nearly half a mile. Its waters 
cover an area of about 475 acres, or three-quarters of a square mile, 
and it drains directly an area of 10 square miles; but since it receives 
the overflow from Loch Chaluim, its total drainage area is 12§ square 
miles. The maximum depth of 157 feet was observed approximately 
near the centre of the main body of the loch. The volume of water 
is estimated at 1366 million cubic feet, and the mean depth at 66 feet. 
The loch was surveyed on October 1, 1902, when the elevation of the 
lake-surface was found, by levelling from bench-mark, to be 267*45 
feet above the sea ; when levelled by the officers of the Ordnance Survey 
on October 26, 1870, the elevation was 267*8 feet above sea-level. 
Loch an Dithreibh includes two basins — (1) a larger deep basin in 
the main body of the loch, and (2) a smaller shallower basin in the 
north-eastern arm, separated by a rise of the bottom on which the 
deepest sounding was 49 feet. The maximum depth observed in 
the small subsidiary basin was 59 feet, and the separating ridge is 
irregular, for a sounding in 21 feet was taken in its central part 
surrounded by deeper water. The 25-feet contour-line is continuous 
from end to end of the loch, coinciding approximately with the outline 
of the loch, but approaching close to the eastern shore off Creag an 
Dithreibh and Creag na Luath-ghaire. The 50-feet area is cut into 
two portions, as already indicated, the main portion approaching close 
to the southern end of the loch and exceeding 1 mile in length. The 
75-feet area i^ nearly a mile in length, and at its northern border the 
lake-floor shows conspicuous undulations, giving to the 75-feet contour- 
