Inverclyde Group
The early Carboniferous succession of the Northumberland Trough, deposited along the southern margin of the Southern Uplands, is distinct from that being deposited in the remainder of England at the time, but very similar to that being deposited in the Midland Valley of Scotland. In recognition of this similarity, recent work has extended the Inverclyde Group and two of its component formations, the Kinnesswood and Ballagan formations, from Scotland into the northern part of the Northumberland Trough.
Kinnesswood Formation
At the base of the Carboniferous succession in the Kinnesswood Formation, locally-developed coarse conglomerates outcrop along the flanks of the Cheviot massif and represent the oldest deposits of the basin fill. These include the Roddam Dene, Ramshope Burn and Windy Gyle conglomerates. Although initially thought to be part of the Old Red Sandstone, they are now generally accepted to be of basal Carboniferous age, despite the absence of diagnostic fossils. The best exposed of these, at Roddam Dene, comprises a mix of beds of conglomerate, sandstone, shale, mudstone and marl of various colours, but is dominated by massive reddish coloured conglomerates of subangular to subrounded, pebble-to-boulder sized clasts of Cheviot andesite with minor amounts of Palaeozoic sediments and rare Cheviot Granite set in a clay-rich sandstone matrix. It is interpreted as the product of ephemeral streams that drained the deeply eroded margins of a Cheviot landmass that was exposed to semi-arid weathering conditions in early Carboniferous times.
The Kinnesswood Formation is named from Kinnesswood, near Loch Leven in Tayside, Scotland, where it is a succession dominated by sandstones, typically with layers of white to pale grey dolomitic or carbonate nodules known as ‘cornstones’. Cornstones form as a result of a fluctuating water table through the soils of semi-arid floodplains.
In Northumberland, such sandstones containing cornstones, for example those underlying the Cottonshope Volcanic Formation and found in the Redesdale valley, were previously assigned to the ‘Lower Freestone Beds’. The sandstones are often coloured red or spotted with brown ochre, and associated with purple, lilac, green and red shales frequently containing ochreous concretions. In most of Redesdale the upper boundary between the Kinnesswood Formation and the overlying Ballagan Formation nearly coincides with the contemporaneous outflow of Cottonshope lavas.
Ballagan Formation (part of the former Cementstone Group)
The formation is named from Ballagan Glen in the Campsie Fells, north of Glasgow. Used throughout Central Scotland for the mudstone, siltstone and cementstone sequence towards the base of the Carboniferous, the name is now also used in northern England to describe the continuation of that succession into the Northumberland Trough.
The Ballagan Formation outcrop forms a broadly arcuate pattern almost entirely surrounding the Cheviot volcanic rocks within the district. East of Chatton the formation is brought up in the core of the Holburn Anticline. It is a sequence of interbedded cementstone, mudstone, limestone and sandstone that is of lagoonal and estuarine origin in the north, but becomes more marine when traced westwards towards Bewcastle. The relative proportion of the different rock types varies across the district and vertically within the formation, which has its maximum estimated thickness of over 600 metres to the south of the River Tweed. The beds range in colour from grey and blue-grey, through green and yellow to brown and pinkish red. The basal cementstones are interpreted as having been deposited in a lagoonal, coastal-flat environment under conditions of high salinity and periodic desiccation.
The cementstones of the district are broadly of two types. Layered cementstones have a sharply defined top and base and some may show signs of internal stratification. They may also have sedimentary features and desiccation cracks. Nodular cementstones show a transition into calcareous mudstones above and below and they form a more or less persistent layer or a row of nodules. The layered cementstones are thought to be primary in origin and the nodular cementstones of secondary origin. The beds are generally less than 0.3 metres thick. The mudstones interbedded with the cementstones are calcareous, silty, poorly bedded and range in thickness from a few millimetres to several metres. Sandstones and siltstone also occur in the sequence and are very variable. Two exceptional limestone developments within the formation are worthy of note. Just south-west of Coldstream the Carham Limestone is a thick-bedded magnesian limestone up to 8 metres thick with a brecciated appearance. 1 kilometre south-west of Rothbury the disused, Glebe Quarry [NU 052 005] provides a section of limestone, some 6 metres thick, widely regarded as one of the finest developments of oncolitic (‘algal’) limestone of the British Carboniferous Period.
