Peebles and Selkirk

Geology is the science that deals with the solid crust of the earth ; in other words, with the rocks. By rocks, however, the geologist means loose sand and soft clay as well as the hardest granite. Rocks are divided into two great classes—igneous and sedimentary. Igneous rocks have resulted from the cooling and solidifying of molten matter, whether rushing forth as lava from a volcano, or, like granite forced into and between other rocks that lie below the surface. Sometimes pre-existing rocks waste away under the influence of natural agents as frost and rain, When the waste is carried by running water and deposited in a lake or a sea in the form of sediment, one kind of sedimentary rock may be formed—often termed aqueous. Other sedimentary rocks are accumulations of blown sand: others are of chemical origin, like stalactites: others, as coal and coral, originate in the decay of vegetable and animal life. For convenience, a third class of rocks has been made. Heat, or pressure, or both combined, may so transform rocks that their original character is completely lost. Such rocks, of which marble is an example, are called metamorphic.

The crust of the earth, in cooling, has contracted into ridges and hollows. The ridges have been worn off and sometimes turned over. Hence it is possible to examine thousands of feet of the earth’s crust from its upturned edges. When one system of rock is laid down regularly and continuously upon another the two systems are said to be confirmable. But if the rocks of the underlying system have been elevated and tilted, or if its surface has been worn away before the younger system has been deposited upon it, the two systems are said to be unconformable. From the order of the strata, from their conformity or nonconformity, and from the characteristic fossils belonging to the various divisions and sub-divisions, we learn that the rocks of Peebles— and Selkirkshires belong to the Palaeozoic or Primary group of rocks, that they are younger than the Cambrian, and older than the Old Red Sandstone and than the Coal Measures of the same group.

When a section of the earth’s crust sinks down in a gap or fracture so that the beds are displaced on each side of the fracture the displacement is called a fault. Two faults run north-east and south-west forming the boundaries of the coalfields of Central Scotland. The southern Uplands lie to the south of the southern line of fault. That is to say, the Old Red Sandstone and Carboniferous strata of the Midlothian coalfield lie up against the Ordovician of Peeblesshire.

The surface of the Uplands is greatly wrinkled and contorted. The ridges of strata are called anticlines, the hollows, synclines. The anticlines may sometimes be so folded over that younger strata lie below older. This has happened in the case of the Birkhill Shales and has consequently made reading of the geological record a difficult task.

In the Ordovician period the strata were laid down in a sea which covered Wales and southern Scotland. In this sea lived plants, and animals of simple form like graptolites, trilobites, corals and starfish. It was a period of intense volcanic activity, and igneous rocks found their way through rents and fissures. A strip of about seven miles broad in the north of Peeblesshire belongs to the Ordovician (Lower Silurian) period. But the greater part of Peeblesshire and the whole of Selkirkshire belong to the period which followed, namely, the Silurian proper (Upper Silurian). This latter period was characterized by the deposition of sediments and limestones in a shallow, quiet, and wide spreading sea; and the life of the period marks a great advance on that of the one previous. For certain forms of insects and fish, and the first representatives of the backboned animals, now began to appear. Till 1852 it was thought that the Silurian rocks of the district were destitute of fossils. But James Nicol, son of the minister of Traquair, showed that greywacke (the older name for Silurian rock) was fossiliferous. Later, Lapworth, then a teacher at Galashiels, established a distinction between the two systems (Upper and Lower Silurian); and, because the latter system is best developed in Wales, named it Ordovician after an ancient Welsh tribe of that district.

After these rocks became land, the downthrow in the trough fault of Central Scotland caused a ridging up of the Southern Uplands into a real mountain range from Girvan to Dunbar, so that the rocks of Peeblesshire, the general dip or inclination of which is N.N.W., plunge in that direction beneath the great Carboniferous basin of southern Scotland not again to reappear till they emerge in a much narrower band under the Grampians. In course of time, however, these mountains of elevation were worn down by sub-aerial forces and the process of denudation was assisted by the fact that the strata of these mountains were anticlines, that is to say, sloped away from the axis of elevation, whereas in mountains built up of synclines, the strata would slope towards the axis of elevation and the mountains would therefore be of more stable equilibrium.

The process may be further illustrated by that the masses would be gradually worn down to an undulating plain, which, having been once more raised to a high plateau of about 3000 feet, the sub-aerial forces renewed their work with increased vigour till the hills and valleys of the two counties assumed practically their present outlines. In this way the hills of Peebles and Selkirk became hills of circumdenudation, i.e. they were, so to speak, dug out not raised up like mountains of elevation. They also became hills of synclinal formation and their valleys valleys of erosion, where, as the erosion continued, the older rocks would be exposed. Thus "the valleys were exalted and the mountains were laid low."

After the Silurian Uplands had been raised the Devonian and Old Red Sandstone strata began to be deposited unconformably in inland seas and lakes bordering on these uplands - unconformably, because the strata of this mountainous surface had been contorted and worn down before the Old Red Sandstone was deposited upon it. It was thus that one formation, raised into dry land, supplied the materials for the next and others in succession. As the Ordovician and Silurian are therefore older than the Old Red Sandstone and the systems that followed it, a great gap exists in the geological history of Peebles and Selkirk up till the glacial epoch, deposits of which they have in abundance.

The fossils characteristic of the Ordovician and Silurian systems are called graptolites from their resemblance to a quill pen. They belong to the order of Hydrozoa. In the Silurian (Upper) the graptolites are nearly all single forms, as, for example, the monograptus. Branched forms as the Didymograptus and Diplograptus are very common in the Ordovician, but quite unknown in the Silurian system. Not only are systems distinguished by their characteristic fossils, but the sub-divisions or groups of systems are themselves distinguished in a similar manner. There are three places in the south-western borders of Peebles and Selkirk where fossils found in black shaly formations could not be identified with the fossils of the Silurian rocks found in the other parts as at Galashiels, where Professor Lapworth first discovered graptolites, and as at Grieston, where Nicol found many specimens of the monograptus. These places were Birkhill, Han Fell and Glenkiln. Two of these groups were identified by means of their fossils with the groups of the Lower Silurian in Wales and the other with the group immediately above it and therefore as belonging to the Upper Silurian.

The district in Selkirkshire where the outcrops of Caradoc, Llandovery and Tarannon rocks (known as the "Ettrick Band ") may best be observed, extends from Craigmichan Scaurs on the south-west of Capel Fell to Berry Bush in Tushielaw Burn, and is bounded on the north-west by the Yarrow and on the south-east by the Ettrick: an area of fifteen miles long by two miles broad and having upwards of fifty exposures. The line of separation between the Upper Silurian to the south-east and the Lower Silurian or Ordovician to the north-west follows the Kingledoors Burn to the Tweed, passes north of Dawyck, west of Stobo, to the Lyne, crossing the Tweed at its junction with that tributary. Passing north of Peebles over Hamilton Hill behind Neidpath it extends along the southern slopes of Makeness Kipps, where, making a return to form a lense-shaped bay, through which flows Leithen Water, it strikes north across the highroad between Innerleithen and Gorebridge, crosses the Gala at Crookston and cuts through the Lammermuirs to Whittinghame. North of the Ordovician area, the rest of Peeblesshire lying north-west of the line of fault (which practically follows the highway from Leadburn to Skirling) including the upper portion of the Lyne valley, belongs to the Old Red Sandstone formation.

Within the Silurian area a thin zone of limestone runs across the valley of the Tweed from Winkston by Drummelzier south-west by Wrae and reappears a little further on at Glencotho. What is perhaps a continuation of this limestone appears at Kilbucho.

Igneous rocks, usually consisting of porphyries, syenites, felstones and dolerites appear in dykes—i.e. vertical walls of igneous rock—coincident as a rule with the direction of the prevailing strike. The most prominent example of felsite porphyry is a group of dykes near Innerleithen on Priesthope Hill, the largest of which extends from above St Ronan’s Mill to beyond Grieston Quarry for about 34 miles. A section is exposed at Walkerburn. A series of outcrops of lava of Arenig age, the oldest exposed rock in the Southern Uplands, beginning beyond Biggar stretches in echelon order along the line of fault as far as Lamancha. These Arenig lavas form the base of the Southern Uplands and would be found anywhere in the region if one could bore deep enough. They appear in the Southern Uplands because the oldest Silurian rocks have been upheaved at intervals all the way across from Ballantrae to the north of Peebles. The base of the Arenig lavas has, however, never been observed.

Many traces of glacial action and glacial drift occur in Peebles and Selkirk, the most important being the boulder-clay (i.e. the clay mixed with boulder stones deposited by the ice-sheet during the Glacial Period), the upper portion of which is often rudely stratified. The lower boulder-clay was mostly swept out of the valleys by the second glacier of this region, which left deposits of boulder-clay thickest in the valleys, but it is to be found up to a height of 1700 feet. It forms sloping shelves or terraces more or less denuded. Examples of these terraces, plateaux or banks, are to be found at Tweedshaws, at Lyne, in the Leithen valley, where also lower boulder-clay has been exposed with interbedded sands and gravels, at Glendean in the Quair valley and at Ettrick Toad Holes. Flutings, or markings due to glacial action on the hill slopes and valleys, are to be seen at Cademuir near Peebles, Kingledoors, Mossfennan, Drummelzier, near which stands Tinnis Castle, surrounded by a fragmentary ravine parallel to the river. In Drummelzier Burn on the slope of Finglen Hill another fragment of a water course seems to mark the bed of the stream which flowed to Tinnis Castle. At Cardrona, Traquair, and in Yarrow, these hollows or trenches of old water courses are also to be found. Terraces formed of banks of sand or gravel drift (left by glacial streams), called "kames," are to be seen in Lyne, at Sheriffmuir near Lyne, in the Meldon valley, and at West Linton. Moraines (deposits left by glaciers) occur at Holylee, where the highway cuts through a terminal moraine, and in Manor, where a very striking series of moraines—one primary and several secondary—form a noticeable feature. In the same valley there is a roche moutonnée, round which the glacier cut its way so deeply that the engineers of the Edinburgh Water Trust failed to find a bottom. There are also moraines near Selkirk, and one, a fine example. on the road to Corbie Linn. Erratic blocks transported by glaciers are not found at a greater elevation than 1100 feet, but they are numerous in the upper grounds of Peebles and Selkirk.

The age and comparative softness of the rocks, the long denudation to which they have been subjected, have produced a striking absence of rugged masses. Another effect of glacial action not so noticeable, perhaps, but worth noting as a confirmation of the trend of the Tweed glacier, is that the western and south-western sides of the hills are always barer and steeper than the opposites sides, due to the forces of glacial action by which formation of "crag and tail" is produced.