The geology of the Tweed catchment, including the action of ice and water on rock, has given the river its characteristic chemistry and landform. Forming the basis of modern geology, the Tweed catchment hosted much of the pioneering work by 18th century geologists. Geologists such as James Hutton, with his internationally known landform at Siccar point on the Berwickshire coast, and Charles Lapworth, with his work on the graptolites and other fossils of the Silurian and Ordovician rocks at Dobb’s Linn, near the Grey Mare’s Tail. Peach and Horne, expounders of plate tectonics in the northwest Highlands, were also active in the Borders.
The catchment of the River Tweed consists of a horse-shoe shaped rim of hills composed of older, harder rocks which surround a relatively flat basin of younger rocks covered with a thick layer of glacial debris. These hills can be divided into two distinct units, the Southern Upland Hills to the north and west and the Cheviot Hills straddling the border to the southeast.
The Southern Uplands
The Southern Upland Fault marks the northern boundary of this range of hills and uplands which encompasses St Abb’s on the Berwickshire Coast, the Lammermuir, Moorfoot and Tweedsmuir Hills. The hill country of the Ettrick, Yarrow and Teviot catchments is also included in this category. The Southern Uplands are formed from strongly folded sedimentary rocks from the Ordovician and Silurian period, 395-500 million years ago, mainly shales, mudstones, slates and greywackes. This geology, coupled with the action of the ice sheets which retreated (ended) around 10,000 years ago, has produced rounded hills and smooth slopes. The scenery is enhanced by deep steep-sided valleys such as those cut by the River Teviot and the Ettrick, Yarrow and Gala Waters. The Loch of the Lowes and St Mary's Loch are formed from a glacially overdeepened basin and were originally one loch. The Leader Water has formed a much broader valley by following a narrow tongue of softer sandstone, which runs northwards into Silurian rocks.
The Cheviot Hills
The Cheviot range of hills is a granite mass surrounded by volcanic lavas formed during the Old Red Sandstone Age some 350 million years old and characterised by smooth rounded hill-tops with steep slopes. Cheviot is noted for its corrie features - crags gouged by glacial activity such as ‘The Bizzle’ and ‘Hen Hole’. The valleys of the Bowmont Water and College Burn cut into the plateau of these hills. A particular feature of the Cheviots is a number of dry valleys formed by melting glacial water as the ice sheets retreated towards the end of the ice age some 15,000 years ago.
The Tweed Basin
Downstream of St Boswells, the Tweed enters the broad rolling plain of the Merse of Berwickshire. Old Red Sandstone rocks underlie much of the western and northern edge of the Merse but, in places, hard igneous rocks are exposed at the surface. The Eildon Hills, which are the remains of mainly acidic lavas intruded into the bedding planes of the Old Red Sandstone strata, are the most famous and conspicuous example of this feature. The two Dirrington Laws, north of Duns, are another example of intruded volcanic rocks forming prominent hills. East of Kelso, the younger calciferous sandstones and limestones from the Carboniferous Period (270-345 million years ago) stretch to the coast, and southwards, along the valley of the River Till, across the Border in England.
Thick layers of glacial deposits cover much of the Tweed Basin, forming a veneer over the bedrock. This material is mainly composed of poorly-sorted boulder clay, formed by the erosive action of ice on bedrock. Fluvio-glacial deposits formed by the action of water beneath, or adjacent to, ice-sheets are commonly sorted into sands and gravels and occur frequently in valley bottoms. The fertile clay-loam soils formed on these glacial deposits support the flourishing agriculture of the Merse. Elongated streamlined hillocks called drumlins, whose long axes reflect the direction taken by the ice sheet, pepper the landscape. Perhaps the most notable of such glacial features is The Kames, just north of Greenlaw which is, in fact, not a kame but an esker formed by deposits from sub-glacial streams.
The lower part of the Tweed, and tributaries such as the River Till and the Eden and Leet Waters, are incised within this terrain of glacial deposits and scarcely come into contact with the rock beneath. The rolling landform of the Merse drops 50-100 feet to the Tweed floodplain and, in most places, the valley through which the river flows is narrow, although wider alluvial expanses occur, such as the areas around Kelso and Coldstream.