Caithness Field Club Main Page

The Boulder Clay of Caithness
J Saxon

In the early clays of the science of geology all the sedimentary rocks had begun to yield up their own particular brand of shell fish. The boulder clay, however, stubbornly refused to yield , any fossils at all and geologists were puzzled by their absence. Nevertheless they searched on. Hugh Miller discovered fragments of shel1 at Wick but he was booked on the ship to take him to Orkney where he was to gather material for his new book "The Asterolepis of Stromness". He had no time to collect them. He wrote therefore to Robert Dick in Thurso telling him of his important find and asking him to collect these marine shells.
The early geologists did not understand the nature of the boulder clay. Hugh Miller believed that it was rock flour produced by the bases of the glaciers scraping over the thon submerged Britain as they moved south from the arctic a theory widely held at that time. If he had realised that the boulder clay was due to the action of ground ice (inlandsis) moving over the surface of the earth he would have regarded a search for marine fossils in the boulder clay as a useless occupation and their discovery might have been delayed for a century or more.

Dick found the boulder clay full of broken shells, some of which he could recognise as similar to those found in British waters at the present day. It is strange that he did not query why stout shells like Turritella were sometimes found whole.
Peach and Horne (1881) spent the whole of their leave one summer in examining the glaciation of Caithness. By this time the true origin of the boulder clay was realised. They detected two distinct ice movements, one sweeping in a north easterly arc from around Latheron on the east coast to around Reay in the north the other moving down slope front the mountains which form the borders of Caithness and Sutherland. The latter movement was expected but the former was a complete surprise. Another surprising thing was that the shelly boulder clay was confined to the area north and east of the arc from Latheron to Reay. In this boulder clay they also found Upper Jurassic and Lower Cretaceous fossils. The conclusion was inescapable.   Ice moving down the slope into the Moray Firth from the Caledonian mountains must have been diverted by some agency so that it turned back on itself to sweep over Caithness to end up in the Atlantic. No one knows what this agency was but the presence of boulders of Scandinavian origin in parts of Orkney suggests that Scandinavian ice, pushing across the North Sea, might have been responsible. The ice sheet, it was presumed, had scoured the molluscs and the Mesozoic fossils from the bed of the North Sea and left them in the boulder clay of Caithness. They sought evidence for the inlandsis entering Caithness on the east coast. Some of the evidence, they claim, can still be seen south of Wick. Just south of the Trinkie Pool detached blocks of Old Red Sandstone can be seen removed from the very edge of the cliff and moved short distances inland. This, they thought, was supporting evidence for an ice movement in this direction.
On the borders between the two ice movements one can often see blue clay presumably derived from the pulverisation of the predominantly dark flagstones interspersed with red clay presumably derived from the granites and the barren red marginal deposits of the Old Red Sandstone, and it thought that these represented the two ice movements. This appears too simple an explanation. In a recent ditch out in a field near the Janetstown road just west of Thurso the excavation cut through (a) top soil (a few tens of millimetres), (b) red friable clay (about one metre), (c) peat (about 150 millimetres). (d) blue stiff clay full of angular fragments of rock. The blue clay is similar to that on the coastal section west of Thurso which is there, full of comminuted shells. The red clay nust supercede the formation of the peat and suggests a different origin from the blue clay. The Quaternarv still holds many secrets from us.

The best shell deposits found by the author were on the east coast of Caithness, often in the steep sides of valleys where there had been landslips. North and east the shells are so broken as, for the most part, to defy identification. Most of the molluscs found still occur in British waters today but some in the list are not recorded. The author is not certain whether they are still extant. If they are, and a similar assemblage is known at the present tine, it may be possible to get a better idea of their environment during the cold Quaternary.

The derived Mesozoic fossils are also of interest as they represent the most northerly Jurassic and Cretaceous in the British Isles. The shores of the east coast of Caithness are littered with them and they are found in the she1ly boulder clay right through to the north coast. They include molluscs, echinoids, zoophytes, cephalapods, and fossil plants.
The author does not know of any record of the northerly boundary of the shelly boulder clay but has found derived Cretaceous fossils as far north Widewall Bay in South Ronaldsay. A search for this northerly boundary is well within the scope of an amateur field worker and could prove a worthwhile project though this may be better undertaken by our Orcadian colleagues. The author has been showrn undoubted Mesozoic fossils reported to have come from Deerness but this awaits more positive confirmation. The shelly boulder clay may extend into the northern islands. A check list of the shells recorded from the shelly boulder clay is appended.
This could serve as a starting point for those who wish to try to add to the list or to help to identify those already found.

Boulder Clay FossilComments
Cyprina islandica
Mactra solida
Mya truncata
Turritella ungulina
Astarte elliptica
Astarte borealis
Astarte compressa
Natica nitida
Natica islandica
Nucella lapillus
Patella vulgata
Veneupis pullastra
Mytilus sp.
Still common in Pentland Firth
Not in British list
Not in British list
Not in British list
Still found in Britain but not in Pentland Firth
Not in British list
Not in British list
Not in British list
Not in British list
Still common in Pentland Firth
Still common in Pentland Firth
Still found in Britain but not in Pentland Firth
Still common in Pentland Firth

Peach, B. N. & horne (1881) The Glaciation of Caithness
Proc. roy. Phys. Soc. Edinb., 6, 316 - 352

Published October 1973