Mearns Drift Group, Quaternary lithostratigraphy, Cainozoic of north-east Scotland
From: Merritt, J W, Auton, C A, Connell, E R, Hall, A M, and Peacock, J D. 2003. Cainozoic geology and landscape evolution of north-east Scotland. Memoir of the British Geological Survey, sheets 66E, 67, 76E, 77, 86E, 87W, 87E, 95, 96W, 96E and 97 (Scotland).
= Mearns Drift Group
The Mearns Drift Group comprises interbedded diamictons, glaciolacustrine silts and clays, and glaciofluvial sands and gravels that are all typically vivid reddish brown in colour and contain clasts that are derived mostly from the andesitic volcanic rocks and red Devonian conglomerates, sandstones and siltstones of Strathmore. The group broadly equates with the southern outcrop of the ‘Red Series’ of Bremner (1916), Sutherland (1984a), Sutherland and Gordon (1993), and Hall and Connell (1991). The deposits were laid down by, or at the margins of, ice that flowed north-eastwards into the North Sea basin from Strathmore (Figure 4). The formations in the group are depicted on Sheet 66E Banchory and Sheet 67 Stonehaven. Deposits occur at least as far north as Nigg Bay (Appendix 1).
Three formations have been identified within the Mearns Drift Group to include the known glacial, glaciofluvial and glaciolacustrine deposits. They are the Mill of Forest Till, Drumlithie Sand and Gravel, and Ury Silts formations, respectively. The deposits are the product of the Main Late Devensian glaciation. Locally, some older units may be present at depth, but unless some intervening materials of a different origin and colour occur, it is difficult to distinguish them. No examples of older units of red till are known, but two distinct phases of movement of the Strathmore ice stream have been deduced by Armstrong et al. (1985) from the Dundee district, an earlier movement directed towards the south-east and a later one towards the east-north-east. This reorientation of ice flow is presumed to have occurred during the Main Late Devensian glaciation, either as the result of an expansion of the Southern Upland ice cap as the glaciation progressed (Sutherland, 1984a), or as a response to Scandinavian ice pushing into the North Sea basin. The presence of red till at the mouth of most of the valleys that drain the southern flank of the eastern Grampian Highlands, such as Glen Clova and Glen Esk (Bremner, 1934b, 1936; Synge, 1956), indicate that in general the Strathmore ice either forcibly pushed back ice occupying the valleys to the north, or that the ice within them had already begun to retreat enabling the Strathmore ice to advance towards the mountains. A more complicated sequence of events appears to have occurred at the Balnakettle site, near Fettercairn (Appendix 1).
There are several notable occurrences of dark bluish grey shelly diamicton (Benholm Clay Formation) underlying red till along the coast to the south of Aberdeen (Campbell, 1934; Auton et al., 2000), for example at the Burn of Benholm site (Appendix 1). It is interpreted as having been deposited by ice moving onshore from the North Sea basin, and hence the Benholm Clay has been assigned tentatively to the Logie-Buchan Drift Group (see below).
Mill of Forest Till Formation
Reddish brown tills laid down by the Strathmore ice stream are assigned to the Mill of Forest Till Formation. These typically cohesive, silty and clayey diamictons contain a mixture of clasts derived from Old Red Sandstone strata and Devonian volcanic rocks, together with some clasts of granitic and Dalradian metasedimentary rocks. Well-rounded clasts of igneous and metamorphic rocks are mostly recycled from Old Red Sandstone conglomerates, whereas more angular clasts are derived from outcrops of Dalradian metamorphic and Caledonian igneous rocks forming the adjacent Grampian Highlands. Some of this ‘Highland’ material may have also been recycled from pre-existing glacigenic sequences during the Main Late Devensian glaciation.
The type section of the Mill of Forest Till Formation occurs in a river cliff (NO 8630 8538), 150 m downstream of Mill of Forest Farm, Stonehaven, where 6 to 7 m of stiff, reddish brown, matrix-supported, silty sandy diamicton crops out beneath 3 m of cobble gravel of the Drumlithie Sand and Gravel Formation (Map 11). The till contains rounded pebbles and cobbles of quartzite, psammite, andesite and feldspathic microgranite, derived mostly from adjacent outcrops of conglomerate (Plate 8b). During the revision survey of Sheet 67 in 1926, Campbell observed a metre-thick unit of grey till at the base of the river cliff.
The Mill of Forest Till Formation is generally less than 5 m thick. Grossly overconsolidated lodgement tills are commonly developed in its basal part whereas friable, sandy to gravelly diamictons (flow tills) up to about 2 m thick commonly occur in its upper part, and locally interdigitate with the overlying Drumlithie Sand and Gravel Formation. One such flow till was recorded in a reference section (BGS trial pit NO77NW1) near Drumelzie (NO 711 790), on Sheet 66E. In another reference section on that sheet (BGS trial pit NO77NE11), at East Mondynes (NO 780 797), a flow till rests on red-brown lodgement till (Auton et al., 1990).
Drumlithie Sand and Gravel Formation
Deposits of the Drumlithie Sand and Gravel Formation were laid down principally as coarsening-upward sequences of sand and gravel at, or near, the margin of the actively retreating Strathmore ice stream, which impinged locally on the coast between the mouth of the River Dee and Stonehaven. Mounds, plateaux and ridges were formed at the retreating ice margin as fans and deltas that were subjected to minor re-advances as the ice withdrew. Eskers are common. Deposits commonly display evidence of slumping, small-scale normal faulting and cryoturbation, and are capped by up to a metre of red-brown sandy flow till.
The sands and gravels contain a mixture of clasts derived from the Silurian and Devonian clastic, volcaniclastic and volcanic rocks of Strathmore together with metamorphic and granitic clasts from the Grampian Highlands. Some of the clasts of crystalline rock have been recycled from Old Red Sandstone conglomerates. For example, the most characteristic durable clasts are well-rounded boulders and pebbles of quartzite from conglomerates. Clasts of soft red-brown sandstone, siltstone, mudstone and decomposed purple andesite are also common, but their relative proportions decrease rapidly away from the Old Red Sandstone outcrops from which they were derived. In its type area, between Auchenblae and Temple of Fiddes (NO 817 818), on Sheet 66E, the Drumlithie Sand and Gravel is typically 5 to 10 m thick. Thickly bedded sands with lenses of gravel are exposed in the partial type section in the Meikle Fiddes esker at Kaim of Clearymuir (NO 798 815) (Map 10).
Ury Silts Formation
The glaciolacustrine Ury Silts Formation consists of characteristic red-brown interlaminated micaceous fine-grained sand, silt and clay. These sediments were laid down in ice-marginal lakes that developed as the Strathmore ice stream retreated south-westwards. Exposures in the Ury Silts Formation are sparse. A partial type section is taken from 14.5 to 18.5 m depth (unbottomed) in BGS Borehole NO88NE4, drilled near the Houff of Ury (Auton et al., 1988), on Sheet 67 (Map 11). This reddish brown silty deposit, which contains sparse isolated pebbles (interpreted as dropstones), is overlain by 1.7 m of moderate reddish brown, friable silty diamicton (flow till). The flow till is overlain by 12.4 m of Drumlithie Sand and Gravel Formation. More typically, there is a gradational contact between the silt and clay unit and the overlying sand and gravel, as in a reference section taken in BGS Borehole NO88NW13, drilled in Craigies Wood (NO 8389 8514), near Stonehaven. The base of the Ury Silts Formation is generally in sharp contact with underlying diamictons of the Mill of Forest Till Formation, or with bedrock.
Relationship between Mearns and East Grampian Drift Groups
The Mill of Forest Till Formation generally rests directly on bedrock, but in some coastal localities, notably at Nigg Bay (Appendix 1), Cove (Jamieson, 1882b) and Findon (Synge, 1963) red till rests on grey till (Maps 9, 11). Both Jamieson and Synge observed gradational contacts between the tills and concluded that they were laid down during the same (Late Devensian) glaciation. The relationship suggests that the direction of ice flow changed from east-north-east to north-northeast as the Strathmore ice stream became dominant. The grey tills contain clasts derived from the west and are correlated here with the Banchory Till Formation of the East Grampian Drift Group (Table 7). At the Burn of Benholm site (Appendix 1), the Mill of Forest Till Formation rests with a sheared (glacitectonic) contact on the Benholm Clay Formation of the Logie-Buchan Drift Group.
Temporary sections at Ury Home Farm (NO 858 881), on Sheet 67, revealed up to 2.0 m of red-brown clayey lodgement till (Mill of Forest Till Formation) overlying up to 2.2 m of moderate yellowish brown sandy bouldery diamicton with clasts of Dalradian metasedimentary rocks and pink granite (Banchory Till Formation). The relationship between the tills suggests that East Grampian ice advanced into the lower reaches of the valley of the Cowie Water prior to advance of the Strathmore ice (Figure 4). In contrast, at Balnakettle (Map 10), near Fettercairn (Appendix 1) and Cantlayhills (NO 874 905), north of Stonehaven (Map 11), sandy diamictons assigned to the Banchory Till Formation overlie the Mill of Forest Till Formation. At both sites, the upper unit is interpreted as flow till, indicating that retreat of the East Grampian ice sheet and Strathmore ice stream was contemporaneous.
Deposits of the Drumlithie Sand and Gravel Formation are locally concealed beneath thin diamictons of the East Grampian Drift Group along parts of the Highland boundary. For example, at Balnakettle (Appendix 1) and Cantlayhills, sheared and contorted beds of reddish brown sand and gravel occur beneath yellow-brown sandy diamicton. At Cantlayhills, the diamicton was formed either from a debris flow directly from the East Grampian ice sheet, or as a landslide following deglaciation. In contrast, at Balnakettle (Plate 26) the sand and gravel has been disturbed by a local re-advance of the East Grampian ice sheet following the withdrawal of the Strathmore ice stream in the area (Appendix 1).