Paraglacial conditions, climate and isostacy control river incision and terrace development; the example of the River Lune, NW England

Abstract

Within Britain, post-last-glacial river terraces are frequently indicators of catchment-wide extrinsic eustatic and isostatic drivers, as the landscape adjusted through paraglacial conditions and latterly experienced temperate climates. Climate drove changes in river discharge and sediment loads at the catchment scale, mediated by local intrinsic controls on terrace formation. The terraces of the River Lune, NW England, are described and related primarily to climatic drivers with a subordinate role for isostacy. Localized drivers include: 1) glacial over-deepening; 2) terrace effacement due to change in the river style; 3) a moraine blocking the river course; 4) the influence of bedrock gorges. Humans may have had a reinforcing effect on climatically-driven terrace formation.

The terrace levels are from highest to lowest: T1 to T3. The T1 level is a degraded, broad, glacifluvial surface; an ice-front braidplain that formed early during deglaciation (c. < 19 ka). The T2 level consists of gravel deposits on straths cut into bedrock, till or the T1 deposits, where the river bed aggraded during the Windermere Interstadial, then incised during the Younger Dryas. The T1 and T2 levels formed during the last glacio-eustatic sea level fall. The T3 level is broadly synonymous with the modern floodplain, which developed from the Early Mediaeval Period, due both to changes in climate and human use of the catchment. In upstream reaches, this level is in the process of abandonment. Major bedrock controls on terrace formation occur at the Lune Gorge and at the Knot Anticline. The latter limited isostatic and eustatic drivers from propagating incision upstream from the modern coast.

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