Before the presentation of residual granite landforms commences, a few terminological issues need to be raised. Nowadays, there is little doubt that the majority of individual medium-scale granite landforms are due to selective subsurface weathering. Twidale (1981a, 1982, 2002) reviewed many early accounts and concluded that selective subsurface weathering and subsequent exposure of unweathered cores to form boulders and inselbergs had been appreciated as early as the end of the eighteenth century. Therefore, the discussion about their origin and significance has inevitably been tied to the increasing recognition of the significance of deep weathering. Outside arid areas there are very few examples of tors and inselbergs, surrounded by a rock-cut platform. The unifying characteristic of all three landforms considered in this section is that they are essentially outcrops of solid rock rising above a surface cut across a weathering mantle, even if the thickness and lithology of the weathering mantle may be very variable. Likewise, granite tors, especially in classic areas such as Dartmoor (Gerrard, 1994a) do not cease to attract the attention of geomorphologists. In the voluminous literature about inselbergs, papers focused on those developed in granite evidently prevail (see the reviews by Kesel, 1973 and Thomas, 1978). Likewise, rock-built residual hills figure prominently in Klimamorphologie des Massengesteine by Wilhelmy (1958). Twidale (1982) in his Granite Landforms considered boulders and inselbergs as two key individual components of granite landscapes and devoted to them almost 100 pages, whereas the other major landforms received only 35. Although none of these landforms is unique to granite, nor even specific to basement rocks, it is perhaps true that the most astounding ones occur within granite areas. Copyrightīoulders, tors and inselbergs (Plates III, IV, V) are regarded as the most characteristic individual geomorphological features of granite landscapes and it is their assemblages extending over large areas that give granite terrains their unmistakable appearance. Des évaluations chronologiques permettent d'estimer à plusieurs dizaines, voire centaines de milliers d'années le temps nécessaire à la transformation des blocs en particules fines. Les processus de météorisation les détruisent avant qu'ils n'atteignent les talwegs. Les résultats indiquent que les blocs libérés par les tors deviennent de moins en moins nombreux et de plus en plus petits à la surface des pédiments quand leur distance par rapport au lieu d'origine augmente. Les méthodes et techniques utilisées sont la granulométrie et la géochimie des roches et des particules fines ainsi que la mesure de l'orientation des blocs. Le lieu d'étude se situe sur une des collines du batholite d'Old Crow, à la frontière internationale entre l'Alaska et le Yukon. Copyright © 2001 John Wiley & Sons, Ltd.Ĭet article montre que les blocs de granite issus de tors se distribuent selon trois zones distinctes sur les pédiments et qu'ils sont réduits en granules, sable et fines à la surface même de ces pédiments. Chronological evaluation suggests that 10s to 100s ka are necessary for weathering to reduce the boulders to fine particles. Weathering processes disaggregate the boulders before they reach the talwegs. Results show that boulders become smaller and less frequent on the pediment as distance from the tor increases. The methods and techniques used were grain size analysis, rock and sediment geochemistry and boulder fabric. The study site is located on a rounded hill of the Old Crow batholith near the Yukon-Alaska border. The boulders are broken down to granules, sand, silt and clay on the surface of the pediments. This article describes how tors calve granite boulders which are then distributed in three zones on the surrounding pediments.
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