A general model of autochthonous blockfield evolution

Two competing models have been invoked to explain the evolution of autochthonous blockfields: the Neogene model, which envisages that blockfield debris was produced by pre‐Quaternary chemical weathering; and the periglacial model, which explains autochthonous blockfields as the product of weathering and frost sorting under cold conditions during the Quaternary. This paper proposes that the evidence favouring both models can be accommodated within a single evolutionary framework. Research demonstrating tor emergence during the Pleistocene and rates of long‐term regolith production indicate lowering of blockfield‐mantled surfaces by at least several metres during the Quaternary. Such lowering implies that regolith covers have undergone continuous or intermittent renewal as surface losses are offset by lowering of the weathering front at the regolith‐rockhead boundary. Depending on initial (pre‐Quaternary) regolith depth and the surface lowering rate, some blockfields may retain inherited Neogene characteristics, but if lowering exceeds pre‐Quaternary regolith depth, a blockfield dominated by the products of mechanical weathering develops. It is proposed that the dominant mechanism operating in the later stages of blockfield evolution has been frost wedging of jointed bedrock at the base of the active layer, a process favoured by saturation of the lower parts of blockfields during seasonal freezeback and possibly by upwards freezing from ‘cold’ permafrost. Copyright © 2010 John Wiley & Sons, Ltd.