216 BATHYMETR1CAL SURVEY OF 



east from Loch Grunavat. An isolated hill, Suainaval, 1403 feet in height, 

 stands at the lower end of the loch on the east side. It is one of the 

 longest lochs in Lewis, measuring 2f miles in a straight line from north to 

 south. The greatest breadth towards the north end is a little more than 

 half a mile. 



Loch Suainaval is a simple basin, with the slope of the bottom steepest 

 from the shore to the depth of 100 feet. The 100-feet contour closely 

 follows the shore-line except at the ends, and the area enclosed by it is 2^ 

 miles in length. From the 100-feet contour to the centre the slope is more 

 gradual, and only two small areas exceed 200 feet in depth. The larger of 

 these areas, in the broadest part of the loch, is one-third of a mile long by 

 one-fifth of a mile broad, and is flat-bottomed, with a greatest depth of 

 212 feet. The lesser 200-feet area is a little south of the centre, is very 

 narrow, and includes the maximum depth of 219 feet. A study of the 

 contours shows that the loch has the U~ sna P e ^ cross-section characteristic 

 of lochs formed in valleys which have been occupied by glaciers. 1 ' The 

 mean depth, 108J feet, is very great, more than three times that of any 

 other loch in Lewis. 



The superficial area is nearly a square mile, or about one-fourth that 

 of Loch Langavat. The volume of water, 2843 millions of cubic feet, 

 shows that Loch Suainaval is the greatest lake in Lewis, being 450 

 millions of cubic feet more than that of Loch Langavat, six times that of 

 Loch Grunavat, and eighteen times that of Loch Trealaval or Loch 

 Fadagoa. The drainage area amounts to nearly 10 square miles. The 

 outflow northward to Loch Stacsavat is by the short river Eyscleit, with 

 a fall of about 1^ feet. The surface was 37*4 feet above sea-level on 

 July 24, 1903, which is about a foot lower than the level found by the 

 Ordnance Survey on October 2, 1895. 



A series of temperatures taken on July 24, 1903, showed, consistently 

 with the great depth of the loch, a greater range than was observed in any 

 other loch in Lewis. The range from surface to bottom was 11'2 Fahr., 

 and the distribution as shown in the following table : 



Surface.... 57 '0 Fahr. 



25 feet 55-2 



50 53-0 



75 50-4 



100 47-7 



200 45-8 



Loch Raonasyail (see Plate LXXXVIII.) is a small loch among the 

 high hills which lie between the south end of Loch Suainaval and the west 

 coast. It occupies a narrow valley between Tahaval, 1688 feet, on the 

 east, and Mealasval, 1885 feet, on the west, which rise in steep crags 

 covered with large and small stones on either side. The loch is of oblong 



* See Collet and Johnston, " On the Formation of Certain Lakes in the Highands," 

 Proc. Boy. Soc. Edin., vol. 26, p. 108 (1906). 



