Abstract Diamond mega-placers, defined as ≥ 50 million carats at ≥ 95% gem quality, are known only from along the coast of southwestern Africa, fringing the Kaapvaal craton, where two are recognized. One is associated with the Orange-Vaal dispersal, the other, to the south, has an uncertain origin. Placers are residual when left on the craton, transient when being eroded into the exit drainage, and terminal. Terminal placers, the final depositories of diamonds, have the greatest probability of being a mega-placer. There are four main groups of controls leading to the development of a mega-placer: the craton, the drainage, the nature of the environment at the terminus and the timing. Cratons, being buoyant, have a tendency to leak diamonds into surrounding basins; however, being incompressible they may have orogens converge onto them resulting in some lost sediment being returned as foreland basin fills. The craton size, its diamond-fertility and the retention of successive kimberlite intrusions that remain available to the final drainage, are significant to mega-placer development. Maximum potential recovery is achieved when the drainage delivering diamonds to the mega-placer is efficient, not preceded by older major drainages and focuses the supply to a limited area of the terminal placer. There should be sufficient energy in the terminal placer regime to ensure that sediment accompanying the diamonds is removed to areas away from the placer site. All conditions should be near contemporaneous and most were satisfied in the Orange-Vaal Rivers-Kaapvaal system and mega-placers were consequently generated.

Abstract Two seminal papers, one from a worker who had insight into tectonics and sedimentation (Kennedy 1958), the other from a classical stratigrapher who had an eye for detail and great experience in synthesizing data (George 1960), present quite different views of the Midland Valley. This difference of opinion, although now in a different form, still exists over both the nature and role of the Midland Valley in Caledonian geology and the nature and significance of its boundaries. The Midland Valley spans the gap between the deeper parts of the Caledonian orogen to the north, where the Dalradian block underwent Cambro-Ordovician burial, metamorphism and cooling and the more superficial but more clearly subduction- related region to the south. Research over the past three decades has assumed that in the south we have the preserved sedimentary record that can be directly related to a mechanism (subduc- tion) that may have provided the required scale and intensity of heat and pressure required to metamorphose and deform the rocks to the north. Establishing the nature of the link between these two regions is critical to an understanding of the Caledonides in the UK, and the Midland Valley of Scotland, being the ground between, is clearly of some significance. However, Lower Palaeozoic rocks of the Midland Valley are poorly exposed being mostly covered by Upper. They are found mainly along its northern and southern margins ( Fig. 5.1 ). This paucity of exposure within the Midland Valley has left much room for speculation about its

Abstract The Lower Palaeozoic accretionary thrust belt forming the Southern Uplands terrane lies to the northwest of the Iapetus Suture and to the southeast of the Southern Upland Fault ( Fig. 6.1 ). The former structure marks the line of closure of the Iapetus Ocean and of collision between the continental blocks of Laurentia and Avalonia; it underlies the Solway Firth and approximates to the line of the Anglo-Scottish border. The latter structure separates the Southern Uplands terrane from the mainly Upper Palaeozoic rocks of the Midland Valley terrane and in particular the Ballantrae Complex. The following sections have been contributed by B. J. Bluck (Ballantrae), P. Stone (Southern Uplands stratigraphy, and provenance) and G. J. H. Oliver (Southern Uplands structure, metamorphism, and geodynamics).

Abstract Old Red Sandstone rocks of the Midland Valley of Scotland record the amalgamation history of blocks widely separated from each other in Ordovician times. As a response to the lateral juxtaposition of the Midland Valley against the Highland block, the Midland Valley, weakened during its long history of igneous activity, was subjected to transtension and transpression, which opened and closed basins resulting in much recycling of sediment and the renewed development of intermittent and sometimes prolific volcanic activity. The Old Red Sandstone comprises two cycles of basin fill: the older (the Lower Old Red Sandstone) is separated by a major unconformity from the younger (the Upper Old Red Sandstone). Each cycle begins with conglomeratic sedimentation in pull-apart basins then fines and petrographically matures upward. The cycles, initiated by faulting, record the decline in tectonic influence and the concomitant reduction in source relief. The sediment source to the Old Red Sandstone is enigmatic. The Dalradian basement to the north and Southern Uplands to the south were, in many places, extensively eroded by Late Silurian–Early Devonian times. The thickest sequence of coarse sediment in the UK, if not in Europe, therefore has no obvious nearby uplift to provide both the sediment and the persistent, substantial slopes required to deliver sediment of that calibre to the basin. To the northeast, the Greenland-Baltica collision had created a major Silurian–Carboniferous (Scandian) uplift. Major river systems draining this mountain belt entered the Lower and Upper Old Red Sandstone basins at a late stage in their development when relief was lowered sufficiently to allow access to them.