342 Peach—On Traces of Glacial Drift 
group lie far out in the sea. They are called Gruna, Bruray, and 
Housay ; there is an excellent harbour here, having two good 
entrances for vessels and boats, and a third into which boats can 
run when the tide answers. ‘These entrances are triradiate, with 
the harbour in the centre, sheltered by the three islands from 
every wind. 
The islands have been the scene of great grinding action, their 
surfaces being all more or less rounded, and bearing the roches 
moutonnées form of knolls, whether composed of granite, gneiss, 
quartz, or limestone,—these rocks being intermingled throughout 
the group. Although corroded by atmospheric action, and the 
limestone-portions much more affected by sea-water, the rounding 
can be everywhere seen. By these agents, ruts, strive, &c., are thus 
generally obliterated; and although they are often further obscured 
by an overgrowth of lichens, I was fortunate enough to find newly 
bared rock near the Mill Cove, on the islands of Housay, from 
which I removed more of the drift and washed the stone; and here 
ruts, striz, and polishing could be seen as plainly as if just done. I 
also found on a gneiss-rock in the highest part of this island some 
deep ruts and scratches, which, with those at the Mill Cove, ran 
E. and W., this being the direction of the channel of the two prin- 
cipal entrances to the harbour which separates Gruna from Bruray. 
The cliff on the north side of the Mill Cove of Housay is about 200 
feet high; the sea breaks on the top of it in heavy gales and tears 
up the rock, and also throws up material from the deep. So great 
is the force, that large blocks are driven far back from the cliff into 
a large semicircular wall, Between this wall and the cliff a deep 
river-like gully is scooped, down which the water rushes again to 
the sea. ‘The water left in the depressions in this gully is brackish, 
and in it Enteromorpha grows. Mr. Jeffreys and myself gathered 
portions of limpets, mussels, periwinkles, rock-whelks, and other sea- 
shells amongst the sand and gravel, both in and on the edge of the 
gully. The whole of the top of this cliff (much of it is now beyond 
the influence of the seas of the present day) is also strewn with 
proofs of similar action, some of the ridges of stones hanging on the 
rounded sides of the hill. All these loose blocks and stones rest on 
rounded knolls and polished rock, all so polished before the burthen 
they now bear was thrown on them. AlthoughI know of many 
grand instances of such comparatively recent ruin, both in Caithness 
and Orkney, this far exceeds them all. Every season, the terrific 
seas which break on the whole of these islands leave tracks of their 
power of the most astonishing kind. Besides the markings on the 
rocks, I met with several deposits of drifted matter in which rounded 
and striated stones were not uncommon. Some of these deposits 
were from 12 to 14 feet in depth. Perched blocks—but not in 
abundance,—some of large size, were scattered over the whole 
group. 
Unst.—Our next move was to North Unst, Balta Sound being our 
head-quarters. Here, too, glacial action was plainly to be seen. The 
serpentine rock has suffered seriously, and although much acted on 
by the weather, the rounded outline of the hills and knolls tells 
