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 algaa. 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^ 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 north-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 algie 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 24 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. 
