THE GLACIAL TILLS OF CAITHNESS

D. Omand

DRIFT THICKNESS
The low-lying nature of much of the county and the evidence of sections available in the major valleys might give the casual observer the impression that Caithness is thickly plastered with drift. This would be a completely misleading picture. The thickest deposits are found on the county boundary at Drumhollistan (30m, 10Oft), at Scrabster (25m, 8Oft) as well as at Gills Bay, Wick, and the re-entrants of Lybster, Latheronwheel (Janetstown) and Dunbeath. At these coastal locations, Drumhollistan excepted, the drift also descends below sea level. In the major valleys of Langwell, Berriedale and Dunbeath the drift has accumulated to great thicknesses, in the case of the Berriedale Water exceeding 30m (10Oft) and clearly the lesser valleys were at one time completely choked with drift. The many drainage and water schemes that have crisscrossed the county in recent years have given ample opportunity to measure the depth of the drift cover away from the larger coastal re-entrants and water courses. A typical measurement of its thickness is in the region of 1m (3ft). On steeper slopes and along interfluves it is much thinner and may be entirely absent e.g. on Ben-a-Chielt the drift is only a few centimetres thick and on the summit of Ben Freiceadain (ND059558) no drift is visible. In the south-east of the county, where a marked scarp and vale landscape occurs, large areas are driftless, e.g. to the south of Wick in the district between the Hill of Miid Clyth (ND286386) and Raggra (ND3144). Around Camster and Loch Stemster (Achkinloch), the hill of Lychrobbie (ND170322) and at Smerral, to the north-west of Latheronwheel there is also an absence of drift. Moreover, it is evident that in many areas peat rests on solid rock, e.g. on Dunnet Head and in parts of the Achairn and Shielton Mosses.

THE DUNBEATH TILL (EARLY LOCAL TILL)
Although the shelly till is commonly accepted as the earliest authenticated drift deposit in Caithness, Tait (1912) had deduced that there was an underlying drift of local origin. Supplementary evidence was suggested by Robert Dick who recorded in the till near Halkirk the presence of a black band, containing shells, separating bands of a brownish colour. Elsewhere (the location is not stated) he saw "cliffs of pulverised granite resting on blue boulder clay and blue boulder clay resting on pulverised granite" (Smiles 1878). Jamieson (1866) noted a similar section in the Berriedale Water where blue-grey boulder clay was found between red boulder clays. In the blue-grey layer he found "nine or ten pieces of shell and a bit of a balanus".

In the course of field work four sites were found and examined: Drumhollistan (ND921653), Watten (ND247547), Latheronwheel (ND191322) and Dunbeath (ND158300) where, in drift sections, a sharp division of colour existed between a dark-brown to blue-grey shelly till and an underlying till of a light-brown colour. Only at Drumhollistan was a water-laid deposit found separating what proved to be two drifts. It consisted of a bed of water-worn stones which was up to 1.5m (5ft) in thickness. No shells or shell fragments were found in any of the four examples of the underlying drift, and also their matrices, unlike those of the overlying drifts, failed to re-act to hydrochloric acid.

As the underlying light-brown drift may have resulted from an ice movement out from the interior it was decided to take stone orientation measurements in it as well as in the overlying shelly drift and to compare erratic distributions from the obtained stones. In the non-shelly drift an orientation close to north-north-east was obtained at Drumhollistan, Watten and Latheronwheel while at Dunbeath a broad scatter of stone orientation was obtained. The stone counts revealed a most marked lithological difference between the non-shelly drift and the over lying shelly drift. At Dunbeath, Latheronwheel and Watten the lower drifts all contain mudstone, the percentages being high at Dunbeath and Latheronwheel which are near the Basement Bed outcrops in which the soft mudstones are found. No mudstones were found in the fabrics for the overlying shelly drift done at Dunbeath, Latheronwheel and Watten. At Drumhollistan the lower drift with its contained migmatites and hornblende is derived from the area to the south and south-west whereas the upper drift has 76 per cent. of its sample of 50 stones derived from the Middle Old Red Sandstone series which lies to the south-east.

TABLE 1

Fabric Analyses of Superimposed Drifts in Caithness

It is worthy of note that in a quarry at Janetstown 3.2Km (2 miles) south-west of Thurso, Peach and Horne (1881) recorded faint striae running northeast to south-west which were almost effaced by striae crossing them at right angles. Crampton et alia (1914) found similar sets of crossed striae, particularly good examples occurring at Loch Stemster (Achkinloch), and attributed those orientated to the north-east to the result of local ice which had spread out on to lowland Caithness prior to ice coming in from the Moray Firth. Other recordings of north-easterly orientated striae (personal observation) were made at widely distributed sites.

Evidence, then, for local ice having invaded Caithness from the interior of the county at least as far east as Watten, is provided by these sections of non-shelly drift underlying the shelly drift. Analysis of the erratic content of the till suggests that this ice movement was likely to have been from the high ground of the Caithness-Sutherland border in a north to north-easterly direction across much of lowland Caithness. Such a hypothesis is in part confirmed by the results of stone orientation measurements and by the faint markings of the south-west to north-east striae which are sometimes crossed at right angles by striae resulting from the subsequent movement across Caithness of ice from the Moray Firth. It is proposed to call this non-shelly till the "Dunbeath till" as this was the area in which it was first seen in section and recorded (personal observation).

TIE LYBSTER TILL (THE SHELLY TILL)
Personal observations suggest that at least two modifications should be made to existing ideas on the shelly till of Caithness, i.e. its extent and the fact that it can by distinguished by colour.

The accepted westerly limit of the shelly till (Crampton et alia, 1914) should be extended at least as far west as the county boundary at Drumhollistan.

Since it was first described (Cleghorn and Smith 1850) the shelly till of Caithness has been credited as being of various shades of blue or grey or any combination of both. Such descriptions are quite misleading because the colour of the deposit varies greatly from one locality to another. At Wick (ND373503) it may be described as blue-grey, in the Houstry Burn (ND155315) a chocolate brown and at Freswick Bay (ND385680) it is a bright reddish-brown. In Sinclair's Bay the brown/red colour of some drift sections might indicate that John o' Groats sandstone lies off-shore. Variation in colour, then, is directly attributable to the hue of the under-lying bedrock. In the vicinity of Wick the flagstones are blue-grey and this colour is imparted to the till, although it is feasible that such a colour found in sections along the east coast may in part be derived from marine deposits dredged up by ice as it came inland from the Moray Firth. Till deposits overlying the John o' Groats sandstone, such as at Freswick Bay, strongly reflect the colour of the underlying lithology which comprises red sandstones, and the dark chocolate brown of the shelly till in the Houstry Burn derives from the local outcrops of sandstone rocks. Lenses of dark grey and reddish brown till occur in sections in Gills Bay and in the Gill Burn, Freswick (ND373672) where ice has traversed outcrops of both dark flagstones and red John o' Groats sandstone. The drift on Dunnet Head is quite instructive in this respect in that on the east side of the headland sections are distinctly grey-blue in colour, but to the west of the headland the few patches of till reflect the brownish-red colour of the underlying Upper Old Red Sandstones. In tills with all these colour variations many good sections occur where shell fragments and occasionally whole shells may be found, e.g. Latheronwheel (ND191322), Lybster (ND245351), Wick (ND373503), Freswick (ND373672), Gills Bay (ND322733) and Strath Burn (ND253514). Some till deposits, irrespective of general colour, weather in the upper part of the sections to a ferruginous tint, which on superficial inspection gives the impression of a separate glacial deposit. In these weathered sections, which may be up to 1.5m (5ft) thick, shell fragments are rare. Good examples of such sections occur at Wick (ND373503), at Keiss (ND351608) and at Gills Bay (ND322733).

When wet, the shelly till becomes a cloying tenacious mass but, on drying, assumes a rock-like consistency so that it is difficult to make much impression on it even with hammer and chisel. It is possible that the high lime content of the till gives it such strong cohesion. Even till which displayed no macroscopic evidence of shells would react vigorously to the application of 4N HCL. Some till sections display lenticles of sand which are occasionally cemented but they impart no stratification to the deposit.

The evidence from Table 1 suggests that the most abundant rock debris in the shelly till has been derived from the Caithness flagstone outcrops and it is likely that the vast amount of clay and silt in the drift sections originated from the same source, although Sissons suggested some of the fine material may have come from the bed of the North Sea (Sissons 1967). It is quite common to find well-rounded pebbles in the shelly drift sections, which is not at all surprising as the ice which deposited this drift must have incorporated many of these as it crossed the sea prior to encroaching on the Caithness landscape. Another possible source could be the conglomerate outcrops.

Of considerable interest to field workers have been the diversity of foreign rocks contained in the shelly till, which are quite unknown in situ in the Caithness lowland. Among these may be mentioned: garnetiferous schist, augen gneiss, felsite, quartzite, oolitic limestone, Jurassic limestone, chalk, chalk with flints, jet and fossil wood (Peach and Horne 1881, Crampton et alia 1914, personal observation). A piece of Jurassic limestone containing an ammonite was found at Dalemore, blocks of fossil wood were found at Dunbeath and a belemnite was taken from the drift in Gills Bay (ND322733) (personal observation). All these fossilferous erratics could have been derived from the Jurassic outcrops occurring on or perhaps off the east coast of Sutherland.

Generally speaking, the uniformity of character of the Middle Old Red Sandstone deposits of lowland Caithness is of little help in determining the direction of ice movement. One noteworthy exception is provided by the distinctive angular conglomerate which outcrops on the coast at Sarclet 6.4Km (4 miles) to the south of Wick. Blocks of this rock have been found in a broad scatter to the north and north-west as far away as Holborn Head on the west of Thurso Bay (Peach and Horne 1881, Crampton et alia 1914, Sissons 1967, personal observation). This evidence, allied to the occurrence of many Mesozoic erratics and marine fauna in the drift, leaves one in no doubt that the general direction of ice movement which deposited the shelly till was on shore from the Moray Firth. As the bore holes into the Leavad erratic revealed fossils of Cretaceous age and Tertiary clay it seems possible that the parent materials of the erratic underlie the sea off the east coast of Sutherland where Jurassic strata occur.

The evidence of striae and stone orientation measurements do not support the generally accepted view that this glaciation was a broad arcuate movement. Along the coast between Berriedale and Wick the striae trend due north or even slightly east of north and so are not in conformity with a south-east to north-west movement. The results of stone orientation measurements at Lybster and Forse support this evidence. However, within 4.8Km (3 miles) or so inland of this coastal zone the striae are well to the west of north, e.g. 320 degrees on the western flank of Ben-a-Chielt. To the north of Wick the trend of the striae is markedly north-west, bearings being commonly of 320 degrees and even 300 degrees on Skirza Head. It is also apparent that as one goes northward in the interior of Caithness the striae commonly have an increased westward orientation e.g. in the Ulbster area, to the south of Wick, the striae are typically due north but on reaching Haster 3.2Km (2 miles) to the west of Wick they trend at 320 degrees to 330 degrees. The stone orientation measurement made in a section at the Achairn Burn is in agreement with a south-east to north-west movement of ice. Measurements made in the vicinity of Thurso confirm this trend of the Moray Firth ice. Bearings of 310 degrees have been recorded for striae at Watten while on the west side of Dunnet Head they were found to trend at 280 degrees to 300 degrees. At Ham the strong north-west movement of ice at this point is made manifest. In the far north-west corner of the county a diagram compiled for a site at Forss shows a broad south-east to north-west alignment. At Halsary in central Caithness a similar stone orientation was found. At Westerdale the stone orientations gave a broad scatter to the east of north, in contrast to the nearby observed striae which trend to the north-west.

Since Croll (1870) advocated the presence of a 'Scandinavian mer de glace' in the Moray Firth which compelled Scottish ice to overflow lowland Caithness, successive authors have developed the theme (Smith 1968, excepted) of an ice barrier to the east of Caithness. Field evidence in Caithness does point to pressure being exerted on Moray Firth ice but, from the increasing westward orientation of striae as one goes northward in the county, it appears that if such a barrier existed, it lay to the north rather than the east of Caithness.

THE REAY TILL (UPPER LOCAL TILL)
West of the generally dark-grey shelly till limit the typical drift section is a chocolate brown to red colour as the drift derived from the igneous/metamorphic complex and Barren Red sediments of the west and south of the county reflects the predominant hue of the solid geology. As the parent material of this area is much coarser in grain than the flagstone and sandstones of lowland Caithness, it follows that drift sections tend to be grittier and less compacted than those of the north-east of the county. Good exposures of this grittier drift occur along the south coast between Berriedale and the Ord of Caithness. Typical among the contained erratics are blocks of breccia, Helmsdale granite and augen-gneiss which all outcrop in the vicinity. A particularly fine section can be viewed on the north coast at the mouth of the Sandside Burn (ND960652) where the drift is overlain by blown sand. This section is close by the village of Reay and so the till has been named the Reay till. Among the erratic blocks found in this section were: granite, quartzite, red mudstone, breccia, mica-schist and diorite. Many of the contained erratics, especially those of granite and gneiss, were rotted and crumbled on touch. In these sections of locally derived drift no shells were found. This confirms the findings of Peach and Horne (1881) and Crampton et alia (1914). Stone orientation measurements in the Reay till at the mouth of the Sandside Burn and the striae in the area indicate that ice here was moving from the south-east to north-west. At Loch More in the interior of the county the direction adopted by ice was from west to east. The considerable carry over of granite and schist found in the thin drift sections lying on Basement Bed rocks near Lochs Shurrery and More testify to this easterly movement of ice. This might suggest that Peach and Horne (1881) and Crampton et alia (1914) were correct in saying that 'local ice' had moved out on to the Caithness plain and there met a greater force of ice which compelled the local one to swing towards the north-west. This interpretation would make the Lybster and Reay tills coeval, i.e. they are different facies of the same glaciation.

EVIDENCE for the AGE of the SHELLY TILL
C 14 Dating
As the shelly till which was deposited by ice from the Moray Firth represents the glacial maximum in Caithness, it was decided to collect a sample of shells from the till in order to obtain a radio-carbon dating. A 10Ogm, (3.5oz) sample of TURRITELIA UNGULINA shells was collected from a drift exposure at Gills Bay (ND323733) by digging deeply into the till and taking specimens that were at least 2m (6.5ft) below the surface of the section. The radio-carbon dating, using the Libby Half-life of C 14, was carried out for me by Professor F. Shotton, Department of Geology, Birmingham University, who separated the sample into two fractions by dissolving off approximately 30 per cent. (i.e. the outer fraction) with dilute acid and dating it as a separate sample from the inner fraction. The outer fraction (dating reference number, Birmingham 179a) was dated as being greater than 34,700 B.P., the inner fraction (dating reference number, Birmingham 179b) being dated as greater than 40,800 B.P. This second dating was established from a bigger sample than the first one and was carried out over a longer period of time. Its result increases still further the minimum age of shells.

This date is of interest when compared with other radio-carbon dates obtained from shelly till in Scotland. Sissons (1967a) obtained a date of >41,100 B.P. from shells found in a gravel layer underlying till at Berwick. At Gardenstown, in Banffshire, Peacock obtained a date of >39,500 B.P. for bivalve shells from the shelly till (Shotton and Williams 1971). The similarity of the minimum ages of the radio-carbon dates of shells suggest that they are related to the same glacial episode and may (according to Sissons' (1967) reasoning) predate a long interstadial.

R E F E R E N C E S