Site 6 Howe of Byth Quarry

Howe of Byth gravel quarry [NJ 839 573] lies at the confluence of a broad north-east trending valley and a meltwater channel orientated north-west to south-east and which now drains the Moss of Fishrie (Map 4). The site, on Sheet 97, is thought to provide evidence of the glaciation of north-east Scotland in both the Late and Middle Devensian (Hall et al., 1995b). The sequence at the site also includes peat of the Late-glacial (Windermere) Interstadial overlain by cryoturbated gravel laid down during the following Loch Lomond Stadial.

The lithostratigraphy of the site was first set up informally by Hall et al. (1995b), but many of the units have been renamed subsequently by Sutherland (1999) in order to adhere more closely to international lithostratigraphical guidelines. As outlined in Chapter 8, the new names have been incorporated into the regional lithostratigraphy only where appropriate (Table 7). The original and new names are given in (Table A1.5) for comparison.

The oldest deposit is the Howe of Byth Gravel Formation, up to 13 m thick (Figure A1.10), which rests on granite, psammite and weathered red arkosic sandstone bedrock. It consists mainly of quartzite cobbles, which show crude subhorizontal bedding and are derived from the local Devonian conglomerates, together with sparse lenses of imbricate pebble gravel and cross-bedded sand. Palaeocurrents were directed towards the south. Brown gravel-rich diamictons, with a silty sand matrix, occur in the upper part of the H owe of Byth Gravel For mation and thicken to the north. In 1997, new sections in the northern part of the quarry showed the unit to comprise up to 8 m of gravel and gravelly diamicton. The beds and lenses of quartzite gravel were discontinuous and 20 to 80 cm thick, dipping gently southwards. The diamicton units were reddish brown in colour and comprised sandy matrix-supported quartzite gravel. Erect pebbles were noted at two locations within these diamictons indicating cryoturbation. The pit had been extended northwards by 1999 and erect pebbles were also observed towards the top of the unit. The gravels could now be seen to occupy a broad depression in the bedrock surface and the lowest beds contained much soft red sandstone similar to the underlying bedrock. The gravels were heavily iron stained with much black 'iron-pan' in the upper 4 m or so. The diamictons were seen to be greenish grey in colour and some of them could be traced for over 200 m southwards.

The Howe of Byth Gravel Formation is interpreted as forming an ice-proximal fan, deposited by meltwater draining southwards, with associated debris flow activity. In the distal parts of the fan, lenses of imbricate gravel and cross-bedded sand suggest formation of ephemeral bars. Although the high-energy, ice-proximal nature of the formation implies that ice was present along the northern coast of Buchan, the only material of nondurable Moray Firth origin observed within it was a single clast of grey mudstone.

Luminescence dating has given ages of 45 ± 4 and 37 ± 4 ka BP for sands within the Howe of Byth Gravel Formation (Hall et al., 1995b). If correct, these imply that the formation is Middle Devensian in age. This is contemporary with the Skjonghelleren Glaciation of western Norway between 42 and 36 ka BP (Larsen et al., 1987) (Figure 43). However, the degree of weathering observed in 1999 may cast doubt on the dating results. Similar developments of iron-pan were present in the gravels at Tillybrex, near Ellon, which are probably pre-Devensian in age (Chapter 8), and have been seen in the oldest gravel unit at Leys, likely to be of OIS 8 age (see below). A problem is that the gravels contain much Old Red Sandstone-derived iron oxide as coatings to clasts. This material is easily mobilised and can give the impression of prolonged weathering. It is noteworthy that the most advanced development of staining and pan is in the proximal part of the fan where Old Red Sandstone material is most abundant in the gravel.

Overlying the Howe of Byth Gravel is the Byth Till Formation. This freshlooking, reddish brown, massive, matrix-supported, silty sandy diamicton is up to 3 m thick, but averages 0.8 to 1 m. Clast lithology is dominated by quartzite, psammite and Devonian sandstone. The Byth Till has a strong west–east clast fabric. The distinctive lithology and fabric suggest that it was deposited by a different ice stream to that from which the Howe of Byth Gravel was derived. The Byth Till is placed in the East Grampian Drift Group and is probably of Late Devensian age.

In the northern part of the quarry, the Byth Till Formation is overlain by up to 3 m of quartzite cobble gravel, the Auchmedden Gravel Formation. In lithology, this unit resembles the Byth Gravel except that it is less weathered. Clast imbrication suggests deposition by meltwater flowing to the south from an ice front to the north; not obviously that which laid down the underlying Byth Till. The Auchmedden Gravel dates from the deglaciation phase of the last ice sheet, while ice remained to the north of the site.

To the south, the Auchmedden Gravel thins and the Howe of Byth Gravel extends close to the surface. The Thinfolds Peat Bed, up to 1 m thick, lies within a shallow basin on the surface of the till. Pollen analysis shows typical Late-glacial (Windermere) Interstadial pollen assemblages. This dating is supported by a radiocarbon age determination of about 11.3 ka BP (SRR–4830) (Table 8) and by luminescence ages of 13 ± 1.4 and 14 ± 4 ka BP. Overlying the peat is a unit of cryoturbated gravel about 1 m thick, the Todholes Gravel Bed, and 1.3 m of Holocene peat (Figure A1.10).

Further notes taken on visits to the quarry in 1999 to 2000 are given in Hall and Connell (2000).

(Table A1.5) Lithostratigraphy at the Howe of Byth.

References