Scotland. Peter Friend

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FIG 49. Cairnsmore of Fleet, 711 m (10 km east of Newton Stewart), viewed from the southeast. This mountain landscape has been created by erosion of the Fleet granite intrusion. (© Lorne Gill, Scottish Natural Heritage)

      As these hot granite bodies were emplaced, their heat baked the surrounding rocks, creating an encircling metamorphosed zone (an aureole) 1 km or, in the case of the Criffel–Dalbeattie intrusion, even 2 km wide. These aureoles are often rich in mineral veins, deposited by hot circulating fluids released by the crystallising granite. Gold, silver, copper, lead and zinc are common, particularly around the Fleet intrusion, and over 60 copper and iron-rich carbonate veins have been located northwest of the Criffel–Dalbeattie pluton.

      Volcanic vents active during the early Devonian are also present in the area, although they are generally poorly preserved. An exception is the large vent at Shoulder o’ Craig, 17 km southwest of Castle Douglas, on the Dee estuary. The headland here is principally made up of a vent-filling intrusion breccia, which consists of Silurian sandstone and siltstone clasts within a basalt (mafic) matrix. Both vent rock and country rock are cut by very potassium-rich dykes, indicating a magma source deep within the mantle. These dykes often have irregular shapes, and one dyke in the area is known as the ‘Loch Ness Monster’ due to its particularly bizarre outcrop pattern. On a regional scale, this area presents a bit of a conundrum, as volcanic vents, mantle-derived dykes and granite plutons, i.e. igneous rocks from all depths within the crust, were intruded around the same time (between around 415 and 400 million years ago, earliest Devonian), and are now seen at the same level of erosion.

      Further north, the late Silurian and early Devonian was the time when a series of basins first began to develop in what would become the Midland Valley, as crustal tension caused movement on the Highland Boundary and Southern Uplands faults. At this time (around 420 to 400 million years ago), Scotland lay in the interior of a large continent some 20 degrees south of the equator, and in this environment the new Caledonian mountains were eroded rapidly because soil-binding plant cover had not yet evolved. Rivers and streams washed the sediment into the developing Midland Valley basins, forming coarse conglomerates, red sandstones and mudstones, collectively called the Lower Old Red Sandstone. Volcanic rocks (associated with crustal extension) are common in the upper 600 m of the Lower Old Red Sandstone, where lava sheets (predominantly andesite) are intercalated with river and lake sediments, mostly sandstones. Today, principal outcrops include a 400 m-thick lava pile underlying the Carrick Hills and a 600 m-thick lava pile in the Dalmellington area (20 and 30 km east of Girvan, respectively).

      The Carrick Hills lava pile is particularly well exposed along the coast around Dunure (10 km southwest of Ayr). This coastal section has been studied for over a century in an attempt to unravel the complex relationships between the lava and intervening sediment; the upper and lower surfaces of andesite (mafic) sheets are often very irregular, with bulbous, finger-like protrusions that extend upwards and downwards into the sediment, or have become detached completely, forming zones of lava pillows. In places, lava engulfs patches of sediment; elsewhere, the lava is surrounded by sediment. The andesite sheets are generally well jointed, and these joints are often filled with hardened sandstone. Despite these contorted relationships, lamination in the sandstones is generally intact, save for a small zone near the contact. Such irregular contacts are thought to result from the sills being intruded into wet, unconsolidated sediment; as hot magma was emplaced, it vaporised water at the magma–sediment contact, fluidising the sediment in a narrow zone next to the contact. This vapour and its entrained sediment then flowed away along the hot contact surface, offering very little resistance to the magma and allowing bulbous protrusions to form. Likewise, the liquid magma could not push directly against the wet host sediment, and so this sediment remains largely undeformed, except at the contact zone. After intrusion, large amounts of water vapour were trapped in sediment enclaves and at contact zones. As the andesite then cooled, it contracted and cracked, often resulting in a sudden decrease of pressure in the sediment. This led to explosive boiling of the water, fluidising the sediment and blasting it along the fractures and cooling joints. Vesicles (cavities formed by gas bubbles) are also very common in the lavas, generally now infilled by minerals such as quartz, agate or chalcedony precipitated by circulating groundwaters.

      By the middle Devonian (400 to 385 million years ago), further earth movements resulted in uplift and erosion of much of the sediment laid down in early Devonian times, and some of the underlying Ordovician and Silurian. The main granite bodies probably became exposed at the surface during this time, as evidenced by the clasts of Criffel–Dalbeattie granite found in Upper Old Red Sandstone deposits in Area 2 to the east. These late Devonian deposits are rare in Area 1, only outcropping near Dalmellington in a thin strip north of the Southern Uplands Fault.

      The Caledonian Mountains had been largely eroded by the start of the Carboniferous, around 360 million years ago, although the Southern Uplands still formed a considerable upland area. Throughout the following 60 million years of the Carboniferous, deposition occurred mostly in the lowlands of the Midland Valley and the Solway Firth basins in marine or coastal-plain environments. Sea levels varied, resulting in the deposition of limestones, sandstones, mudstones and coal, often arranged in ‘cycles’ of varying layers, as shallow seas and river estuaries gave way repeatedly to swampy forests. Towards the end of the deposition of the Lower Carboniferous, the Southern Uplands had been sufficiently lowered by erosion to be breached by the sea along what is today Nithsdale, and the Midland Valley and Solway Firth basins were linked. Coal deposits were laid down under swampy conditions in the Carboniferous, and are today found around Sanquhar and Thornhill and in the larger Ayr Basin. These sedimentary basins were defined by numerous northwest-trending normal faults. The Carboniferous was also a time of renewed igneous activity, after the quiet of the СКАЧАТЬ