224 BATHYMETRICAL SURVEY OP 



over, and despite the great amount of organic debris resulting from the 

 myriads of animals and plants living in the water, as well as sediment 

 brought in by the streams, the stones were clean, or there was only a thin 

 slimy film due to the growth of diatoms and other algae. How is the 

 clean paved bottom and the absence of peaty deposit so general at the 

 bottoms of these lochs to be accounted for ? Is all the sediment derived 

 from every source carried off by the ordinary slow current of the loch, and 

 is the paved bottom an original and permanent feature ? It does not accord 

 with experience of shallow lochs elsewhere to suppose this. Such lochs 

 commonly silt up, and become overgrown with weeds, and are converted 

 eventually into marshes. If these lochs of Orkney are silting up in the 

 usual way, why the clean bottom and freedom from deposit ? An explana- 

 tion may be found in supposing that the lochs are violently agitated to the 

 very bottom during gales, the stones re-arranged on the top of the latest 

 formed mud, and the material in suspension in the water carried off during 

 spates. 



Loch of Stenness (see Plate XC.). The Loch of Stenness is a large 

 sheet of salt water, measuring nearly 4 miles long and 1 J miles broad, and 

 is about 2 miles north-east of the town of Stromness. The surrounding 

 heather-clad moorland abounds in monuments of ancient peoples. In 

 places the action of the waves has worn the shores into very low cliffs of 

 rock or gravel, but in general the slope is gentle to the water's edge. The 

 axis of the loch runs nortli-west to south-east, with a slight sigmoid 

 curvature. The greatest breadth is in the centre, where a broad bay run- 

 ning to the south-west branches into arms running to north-west and 

 south-east. In the southern bay the tide enters from the Bay of Ireland, 

 under the Bridge of Waith. Though the channel is broad, and the access 

 free, the level of the loch is but little affected by the tides, which indicates 

 that the bar is but little below ordinary high-water level. At the eastern 

 extremity the loch communicates with the Loch of Harray, under the Bridge 

 of Brogar. During the whole of our stay in the islands the two lochs never 

 differed measurably in level, though a current could be seen in one direc- 

 tion or the other. Marine algae grow throughout the loch, and the fauna is 

 marine. 



The Loch of Stenness is flat-bottomed, and has a mean depth of 10^ feet, 

 and a maximum depth of 17 feet, near the south-eastern extremity. The 

 superficial area is 2J square miles, and the volume of water 716 millions of 

 cubic feet. The drainage area, including the Loch of Harray and many 

 small lochs, measures 60 square miles. Apart from the inflow at the 

 Bridge of Brogar, only a few small burns enter the loch. The surface at 

 the date of the survey (August 19, 1903) was 3'6 feet above sea-level. 

 Sir Walter Scott refers to both lochs (Stenness and Harray) as the Loch 

 of Stenness. 



The surface temperature on August 19, 1903, was 58'0 Fahr., and on 

 August 20, 60-2. 



