Elsewhere in Area 1, the ground is much lower-lying, and the effects of glacial erosion are not so obvious. This ground, already low at the end of the Tertiary, has been further lowered and smoothed by the passage of ice. Where hills are present, they tend to have been rounded by ice scouring, and often have a streamlined shape. An excellent example is the island of Ailsa Craig (Fig. 50). Although the island has been considerably modified by subsequent marine action (which produced its precipitous cliffs, discussed below), its overall shape is round, but elongated from north to south, reflecting southerly ice flow. The granite has clearly resisted erosion much more effectively than the soft Permo-Triassic sandstones into which it was intruded. Ailsa Craig was positioned in the path of many different ice streams during the Devensian glaciation, and these ice streams carried blocks of Ailsa Craig microgranite in the direction they were flowing. As the microgranite has a very distinctive composition, these blocks are easy to identify, and they have proved very useful in tracing flow directions of the last ice sheet across Britain. Indeed, the Ailsa Craig boulder train is one of the most famous and largest in the British Isles, extending south across the Irish Sea and parts of England and Wales as far as Pembroke, and westwards to Ireland.
North of the Southern Uplands Fault, the relatively soft sedimentary bedrock of Devonian to Permian age has generally been heavily weathered and eroded by the passage of ice, forming a low-lying landscape. However, the sedimentary units are punctuated by horizons of lava and numerous plugs of fine-grained igneous rock, only the largest of which are shown in Figure 47. As a result, the generally low-lying landscape is frequently interrupted by rounded hills, underlain by the more resistant units. This is well illustrated in the Straiton area (20 km northeast of Girvan), where the craggy hill tops of Bennan Hill and Craig Hill are underlain by more resistant Devonian lavas, with Devonian sediments underlying the gentler slopes of the surrounding area. Likewise, Mochrum Hill (Fig. 52), near Maybole, is underlain by the eroded 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