Abstract In the early 1830s Charles Lyell was convinced that much of western Europe had been submerged during the Pleistocene by cold seas strewn with icebergs; the relicts of whose loads of rock and mud occurred on land as boulder clay and erratic blocks. Swiss scientist Louis Agassiz disagreed, considering in 1837 that these were the products of deposition by a great ice sheet. Archibald Geikie realized by 1863 that Lyell was wrong. Mountain glaciers had carved the topography of Scotland and other parts of the UK, feeding an ice sheet that left glacial erratics behind when it melted away. He hoped, in vain, to change Lyell’s mind. Archibald Geikie’s mantle passed to his brother James, who compiled evidence from around the world to demonstrate the correctness of his brother’s thesis. It was published in 1874 just before Lyell died still arguing for the correctness of his iceberg theory, which gave us the word ‘drift’ for the unconsolidated deposits mantling the UK. Even so, by then Lyell had gone some way – no doubt partly influenced by the Geikies – to accepting that in certain instances glacial action had, indeed, moved large erratic blocks – locally even uphill, as in the Jura.

Continental crust subjected to horizontal contraction in convergent settings deforms in a variety of styles. In many instances, it is useful to consider the deforming crustal sections in terms of crystalline basement rocks underlying incipiently undeformed sedimentary strata. Three deformation styles are commonly found in such settings. The structural style referred to as thin-skinned tectonics encompasses a stack of thrust sheets composed of non- or weakly metamorphic sedimentary rocks. The associated thrust faults usually level off in a mechanically weak décollement horizon along which a substantial amount of displacement occurs in the course of the formation of the fold-and-thrust belt. Thrust faults may also cut down into the crystalline basement and level off a few kilometers beneath the basement-cover interface. The term basement-involved thin-skinned tectonics is proposed to describe this style of continental contraction. This style, too, is characterized by stacks of thrust sheets. In many cases however, such nappe stacks are overprinted by pervasive folding of a crust thermally weakened by magmatic activity or regional burial metamorphism. Thick-skinned tectonics seems less common. This style implies that thrust faults cut across the entire upper crust (and possibly the lower crust). The associated continental contraction is smaller, and the ensuing deformation is characterized by warping of the basement-cover interface. Displacements accumulated in a major basal detachment horizon may connect into mantle by means of a subduction zone. However, under elevated temperatures, pervasive deformation of the hanging wall and footwall rocks may compensate large displacements over relatively short distances. Thin-skinned fold-and-thrust belts are common on both sides of collisional orogens. Noncollisional orogens tend to be more asymmetric.