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Denudation mechanisms differ fundamentally between limestone and silicate rock types, which are subject to very different rate thresholds and enhancers/inhibitors. Silicates are removed largely by erosion, the mechanical entrainment and transport of particles. This is a relatively high energy, and highly episodic, process which occurs only when a minimum threshold flow velocity is exceeded; it is inhibited by vegetation cover and favoured by strongly seasonal runoff. Limestone is removed largely by chemical dissolution at a rate directly proportional to runoff. Dissolution is a relatively low energy process that can occur at any flow velocity or in static water; in general it is enhanced by vegetation cover and non-seasonality of runoff. These contrasting factors in the denudation of silicates versus limestone can produce strikingly uneven rates of surface lowering across a landscape, sometimes akin to the well known 'tortoise and hare race', where the slow and steady denudation of limestones may in the long term exceed the sometimes rapid, but often localized and episodic, erosion of silicates. Prolonged exposure of limestone to a humid temperate climate in a tectonically stable environment produces lowrelief corrosion plains in which limestone uplands are anomalous and, in most instances, due to recent unroofing from beneath a siliciclastic cover. In a highly seasonal or semi-arid climate almost the exact inverse may develop, with 'flashy' runoff and sparse vegetation favouring erosion rather than dissolution. Even under a constant humid climate progressive unroofing of a thick limestone unit within folded siliciclastics may lead to a topographic inversion over time, with the limestone outcrop always forming a topographic low flanked by siliciclastic uplands. Valleys will be initiated on anticlinal crests, where the limestone is first unroofed, but progressive lowering of the limestone causes these valleys to migrate to their final position in the synclinal troughs. In humid climates isostatic compensation in response to slow, but continuous, denudation of extensive limestone outcrops may be a significant factor in the development of relief on adjacent, more slowly eroding, silicate outcrops.
Denudation mechanisms differ fundamentally between limestone and silicate rock types, which are subject to very different rate thresholds and enhancers/inhibitors. Silicates are removed largely by erosion, the mechanical entrainment and transport of particles. This is a relatively high energy, and highly episodic, process which occurs only when a minimum threshold flow velocity is exceeded; it is inhibited by vegetation cover and favoured by strongly seasonal runoff. Limestone is removed largely by chemical dissolution at a rate directly proportional to runoff. Dissolution is a relatively low energy process that can occur at any flow velocity or in static water; in general it is enhanced by vegetation cover and non-seasonality of runoff. These contrasting factors in the denudation of silicates versus limestone can produce strikingly uneven rates of surface lowering across a landscape, sometimes akin to the well known 'tortoise and hare race', where the slow and steady denudation of limestones may in the long term exceed the sometimes rapid, but often localized and episodic, erosion of silicates. Prolonged exposure of limestone to a humid temperate climate in a tectonically stable environment produces lowrelief corrosion plains in which limestone uplands are anomalous and, in most instances, due to recent unroofing from beneath a siliciclastic cover. In a highly seasonal or semi-arid climate almost the exact inverse may develop, with 'flashy' runoff and sparse vegetation favouring erosion rather than dissolution. Even under a constant humid climate progressive unroofing of a thick limestone unit within folded siliciclastics may lead to a topographic inversion over time, with the limestone outcrop always forming a topographic low flanked by siliciclastic uplands. Valleys will be initiated on anticlinal crests, where the limestone is first unroofed, but progressive lowering of the limestone causes these valleys to migrate to their final position in the synclinal troughs. In humid climates isostatic compensation in response to slow, but continuous, denudation of extensive limestone outcrops may be a significant factor in the development of relief on adjacent, more slowly eroding, silicate outcrops.
International audienceThe structural evolution of the English Channel area is controlled by structure and particularly by the pre-existing Cadomian and Variscan crustal discontinuities, which have been reactivated repeatedly in post-Variscan times. They controlled the crustal subsidence that produced basin development in the Mesozoic, prior to the sea-floor spreading in the North Atlantic region. They were then reactivated during the Cenozoic compression and basin inversion. The English Channel development is ascribed to mid-Tertiary differential uplift (Oligocene to Miocene). During late Tertiary to Quaternary times the Channel displays characteristics of a tectonically controlled fluvial basin periodically invaded by the sea. At the lithospheric scale, the Channel can be considered as an active intraplate area influenced by the NW–SE ‘Alpine push', the NW–SE ‘Atlantic ridge push' and glacial rebound stresses
Uncertainties about landscape evolution under cold, nonglacial conditions raise a question fundamental to periglacial geomorphology: what and where are periglacial landscapes? To answer this, with an emphasis on lowland periglacial areas, the present study distinguishes between characteristic and polygenetic periglacial landscapes, and considers how complete is the footprint of periglaciation? Using a conceptual framework of landscape sensitivity and change, the study applies four geological criteria (periglacial persistence, extraglacial regions, ice-rich substrates, and aggradation of sediment and permafrost) through the last 3.5 million years of the late Cenozoic to identify permafrost regions in the Northern Hemisphere. In limited areas of unglaciated permafrost regions are characteristic periglacial landscapes whose morphology has been adjusted essentially to present (i.e., Holocene interglacial) process conditions, namely thermokarst landscapes, and mixed periglacial-alluvial and periglacial-deltaic landscapes. More widespread in past and present permafrost regions are polygenetic periglacial landscapes, which inherit ancient landsurfaces on which periglacial landforms are superimposed to varying degrees, presently or previously. Such landscapes comprise relict accumulation plains and aprons, frostsusceptible and nonfrost-susceptible terrains, cryopediments, and glacial-periglacial landscapes. Periglaciation can produce topographic fingerprints at mesospatial scales (10 3 -10 5 m): (1) relict accumulation plains and aprons form where long-term sedimentation buried landsurfaces; and (2) plateaux with convexo-concave hillslopes and inset with valleys, formed by bedrock brecciation, mass wasting, and stream incision in frost-susceptible terrain.
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