18 BATHYMETEICAL SURVEY OF 



The volume of water is estimated at 21 million cubic feet, and the mean 

 depth at one-half the maximum depth. The loch was surveyed on 

 October 11, 1902, when the elevation of the lake-surface was found to be 

 112*8 feet above the sea, which is almost identical with the elevation 

 determined by the officers of the Ordnance Survey on December 12, 1870, 

 viz. 112-9 feet above sea-level. The surface temperature on the date of the 

 survey was 46 -0 Fahr. 



St. John's Loch (see Plate V.). St. John's Loch lies a little over half 

 a mile to the north-east of Dunnet Bay, into which its overflow is carried 

 by the Burn of Dunnet. It is a small shallow loch ; the western end is 

 stony, and here many shells of lamellibranchs and gasteropods were 

 found on the shore, having been thrown up by the wind. The loch is 

 subcircular in outline, with a maximum diameter from east to west of less 

 than a mile, the maximum breadth from north to south exceeding half 

 a mile, its waters covering an area of about 195 acres, or nearly one- 

 third of a square mile. The maximum depth of 7 feet was observed in the 

 south-eastern part of the loch. The volume of water is estimated at 38 

 million cubic feet, and the mean depth at 4| feet. The loch was surveyed 

 on October 10, 1902, when the elevation of the lake-surface was found to 

 be 71 '3 feet above the sea ; when levelled by the officers of the Ordnance 

 Survey on February 4, 1871, the elevation was 72-1 feet above sea-level. 

 The highest drift-mark observed was a foot above the level of the water on 

 the date of the survey, and it was stated that the water might fall to the 

 extent of 1| feet. 



St. John's Loch is a shallow flat-bottomed basin, the deep water 

 occupying the eastern half of the loch, the deepest cast in 7 feet having 

 been taken about 100 yards from the southern shore and less than 400 

 yards from the eastern end. The area of the lake-floor covered by more 

 than 5 feet of water is about 114 acres, or 58 per cent, of the total area. 

 The temperature of the surface water on the date of the survey was 46'4 

 Fahr. 



