Scotland. Peter Friend

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СКАЧАТЬ remains of a large Devonian volcanic vent, around 1 km in diameter. The vent is filled with agglomerate (coarse angular blocks of volcanic material), which has resisted erosion to form the prominent, rounded hill, whilst the surrounding Lower Old Red Sandstone is much softer and lower-lying. The sandstone in this area is feldspar-rich, and has weathered to produce particularly fine arable soils. Younger volcanic rocks are also common, such as the Permian, agglomerate-filled volcanic neck underlying Patna Hill, just northeast of Patna. There are more than 20 such vents in the Patna–Dalmellington area, many of which are responsible for small topographic features. As these intrusions are often basaltic (mafic), they have weathered to produce nutrient-rich soils, the so-called ‘Green Hills’ of Ayrshire.

      In places, prominent hard bands within the sedimentary rocks are also associated with rounded, glacially scoured hills. An example is the ‘Big Hill of the Baing’ southeast of Straiton (20 km northeast of Girvan), an elongated, faulted ridge of Ordovician boulder conglomerate. More extensive outcrops of this conglomerate occur in the Girvan–Ballantrae area, where, along with the Ballantrae Complex, they underlie higher, hillier ground than the softer rocks further south.

      Landscape modification by glacial deposition

      Much of Area 1 is relatively low-lying, and here the effects of glacial erosion are more subtle than in the high ground of the Southern Uplands: the ground level was lowered, pre-existing Tertiary valleys were deepened and the low hills were moulded and streamlined. Equally important in the formation of today’s landscape in these lowland areas was glacial deposition: on deglaciation, great thicknesses of till were deposited and today glacial till, sand and gravel mantles much of the lowlands. These deposits have a range of surface forms, including eskers, kames, outwash terraces and, in particular, drumlins.

      Drumlin swarms are important landscape features throughout the lowlands of this Area, tending to broadly correspond with the arrows on Figure 53. They mantle much of the Rhins of Galloway, the Machars, the Glenluce, Ballantrae and Girvan districts and Nithsdale. They also make up much of the land surface of the Midland Valley, being responsible for the rather intriguing, ‘hummocky’ texture that is so characteristic when viewed from the air, or on a simple hill-shade map (Fig. 52). The drumlin swarms in these areas produce a distinctive landscape of low hills, typically around 30 m high and 300 m long, all oriented in the same direction and with similar shapes – blunt at one end and tapered at the other, rather like an egg. This streamlined shape is produced by deposition at the base of a flowing glacier: drumlins often have a core of rock or glacial till, and as sediment-laden ice flows over these obstructions, material is deposited downstream of the core, where it is relatively sheltered from ice erosion. As this process repeats itself, a streamlined mound is gradually produced, with a tapered end pointing downstream and a blunt end pointing upstream. One is aware of the whaleback shape of the drumlins that make up these swarms from the ground, but an aerial view allows the best appreciation of their three-dimensional streamlined form. Excellent examples are seen, for example, around Newton Stewart and in the New Galloway district. Smaller swarms are also present in the uplands.

      The broad Carsphairn Valley cuts across some of the highest ground of the Southern Uplands, with the Loch Doon hills to the southwest and the Cairnsmore hills to the northeast. Reconstructions of former ice-flow directions in the valley indicate that, during the Late Glacial Maximum, a northeast/southwest ice divide was located across its central part, passing from Cairnsgarroch summit through Craig of Knockgray to the Cairnsmore Hills. The thickest and most extensive till deposits present in the Carsphairn Valley are found around the area of this ice divide, which seems somewhat contradictory. Horizontal ice flow is minimal at ice divides, and the till cannot therefore have been deposited when the divide existed, so the source and age of this till is an interesting question. The answer seems to be that the till was deposited during or before the glacial maximum, during the growth of the Late Devensian ice sheet. At the start of the Late Devensian glaciation, ice would have initially accumulated in the corries and trough heads northeast and southwest of the Carsphairn Valley. As the glaciation advanced, these glaciers expanded and finally converged in the valley bottom, and as their flow was impeded till would have been deposited. During the subsequent glacial maximum, the preservation of this till beneath great thicknesses of ice is likely due to its location under the ice divide, as although the ice sheet expanded and thickened, the slow rates of ice movement meant the ice had little erosive power here.

      This till, deposited during the growth of the Late Devensian ice sheet and preserved under the ice during the glacial maximum, was then remoulded during a late stage of glaciation into a set of interesting landforms – rogen, or ribbed, moraine, which consists of sinuous, 20 m-high, elongated ridges that run perpendicular to the valley axis. The mechanism by which these till ridges formed, perpendicular to the down-valley direction of Devensian ice flow, is another interesting point. A likely scenario is that the rogen moraines represent ridges of sediment produced by thrusting (by compression) or fracturing (by extension) at the base of the ice sheet. For this to happen, the flow speed of the lower part of the ice must have varied downstream: a sudden speeding-up would produce fracturing by extension; a sudden slowing-down, such as upstream of an obstacle, would produce thrusting by compression. This would have been most likely to happen during a late stage of ice-sheet deglaciation, when faster, more concentrated flow occurred within the main valleys. The most recent episode recorded by this till involved the drumlinisation of the rogen moraine, as the original landforms became elongated down-valley to varying degrees.

      Important amounts of sediment were also deposited by sediment-charged meltwaters flowing out from retreating glaciers, referred to as glaciofluvial deposits, and present in a variety of forms. Sediment may accumulate in channels, ponds and lakes trapped between lobes of glacier ice or between a glacier and the valley side. Where such sediment has a ridge or mound form, it is termed a kame; where it is a flat-topped mound, it is termed a kame terrace, and is likely to have been deposited in a lake. When sub-glacial meltwater drains through tunnels, the sub-glacial stream may deposit sediment as a surface stream would, but confined to the tunnel. The result is a long sinuous ridge of gravel, termed an esker. Outwash plains often build up downstream of the melting glacier, large plains of poorly sorted, stratified sediment deposited by braided streams. Sequences of terraces are often seen in these plains, formed by river incision. Kettle holes are also common features, formed by blocks of ice that become buried in outwash sediment, and then melt to leave behind a depression. Many of these kettles have been infilled with sediments, particularly peat, during the post-glacial times, but some are still visible today as small isolated lakes or deep water-filled depressions in boggy areas that were once the low-lying outwash plains.

      A famous example of ‘kame-and-kettle topography’ is found in Nithsdale, north of Dumfries. Particularly on the eastern side of the valley, there are many short, linear glaciofluvial ridges separated by depressions and hollows. The relative relief between ridge crest and depression is usually between 8 and 25 m, and the ridges are relatively short, with very few being over 500 m long. The extensive gravel pit at Kilblane, for example, is developed in three such kame ridges. The coarse sediment that makes up these ridges, and other linear kame ridges in this part of Nithsdale, was probably deposited in meltwater channels that flowed between ice-cored ridges parallel to the ice margin. As the ice ridges melted, the sediment-filled channels became inverted to produce the kames seen today. Kame terraces are also seen on both sides of the Nith, with the best developed just east of Duncow (8 km north of Dumfries) at an altitude of around 55 m. Further north, in the mid-part of the Nith valley (south of Thornhill), a similar kame-and-kettle topography is seen. The glaciofluvial deposits of Nithsdale account for the rather large number of sand and gravel pits seen just north of Dumfries, now often flooded. Glaciofluvial deposits are relatively common elsewhere in Areas 1 and 2, such as in many of the valleys on the south side of the Southern Uplands and in the area around Stranraer.

      The mapping of glaciofluvial deposits in the Nith Valley has allowed the reconstruction of the pattern of glaciofluvial drainage which developed during a late stage СКАЧАТЬ