256 BATHYMETRICAL SURVEY OF 



after heavy rains. The two lochs trend east and west, and the outflow 

 from Loch na Creige Duibhe passes into Loch Mama, and thence by 

 the Gleann Mama into Loch nan Uamh. 



Loch Mama (see Plate LVL). Loch Mama is over one-third of a 

 mile in length, one-eighth of a mile in maximum breadth, and one- 

 twelfth of a mile in mean breadth. Its waters cover an area of about 

 17 acres, and it drains directly an area of two-thirds of a square mile, 

 but since it receives the outflow from Loch na Creige Duibhe its total 

 drainage area is over 2 square miles, an area seventy times greater than 

 that of the loch. Nearly 40 soundings were taken, the maximum depth 

 observed being 44 feet. The volume of water contained in the loch is 

 estimated at 11 million cubic feet, and the mean depth at 14 J feet. The 

 loch was surveyed on July 11, 1902, and the elevation of the lake- 

 surface above the sea was determined from spot-levels as being 359 

 feet. It forms a simple basin, the deepest part being found towards 

 the east end. The areas between the contour-lines, and the percentages 

 to the total area, are as follows : 



bo 10 feet 8'0 acres 46 '9 per cent. 



10,, 25 6-4 ,, 37-5 



Over 25 , 2'6 , 15 "6 



170 , 100-0 



Loch na Creige Duibhe (see Plate LVL). Loch na Creige Duibhe is 

 four-fifths of a mile in length, one-eighth of a mile in maximum breadth, 

 and one-fourteenth of a mile in mean breadth. Its waters cover an 

 area of about 36| acres, and it drains an area twenty-four times greater, 

 or about 1 \ square miles. Over 70 soundings were taken, the maximum 

 depth recorded being 93 feet. The volume of water is estimated at 

 52 million cubic feet, and the mean depth at 32 \ feet. The loch 

 was surveyed on the same day as Loch Mama (July 11, 1902); the 

 elevation of the lake-surface above the sea, from spot-level and by 

 comparison with Loch Mama, was found to be 359*7 feet. An inspection 

 of the map shows Loch na Creige Duibhe to be (like Loch Mama) a long 

 narrow basin of very simple conformation. It is much deeper than Loch 

 Mama, and the deeper water approaches nearer to the west than to the 

 east end, that is to say, nearer to the alluvial cone separating the two 

 lochs. A similar state of matters has been noted in the case of Lochs 

 Voil and Doine in the Forth basin, formerly a continuous loch, now 

 divided into two portions by the deposition of material brought down 

 by the river, where deep water approaches close to the dividing 

 promontory of land on both sides. * 



The areas between the consecutive contour-lines drawn in at equal 



* See p. 9. 



