Summary
Glacial tills in coastal Northumberland deposited by last glaciation (Devensian) ice (c. 25,000 years B.P.) exhibit a reddened post-depositional weathering profile up to 8 m thick, which has previously been misinterpreted as a separate stratigraphic unit, or ‘Upper Boulder Clay’.
Weathering profiles are developed in grey lodgement tills that form an extensive coastal lodgement till plain deposited beneath a wet-based ice sheet. The tills contain intraformational sands, gravels and laminated clays deposited in subglacial meltwater channels and lakes. These sediments have often been partially eroded and incorporated within overlying till resulting in lateral variations in the depth of post-depositional weathering. Sand and gravel lenses, for example, act as drainage layers within the till thereby increasing the depth and degree of weathering and often forming a sharply defined lower boundary to the weathering profile. These features are seen as an apparent tripartite stratigraphy consisting of an upper red till, middle sands and gravels and a lower grey till. Where the depth of the till section is less than between 2 and 3 m only a single reddened weathered till is found.
Weathering takes the form of oxidation followed by leaching of carbonates and is characterized by colour change, increasing content of rotten boulders, and prismatic gleyed jointing. Within the weathering profile four distinct weathering zones can be recognized. Clay and moisture contents increase with degree of weathering giving rise to systematic zonal changes in Atterberg indices and shear strength parameters.
Deep weathering (≏ 8 m) of Devensian till is limited in its geographical distribution to eastern Britain and can be related to the glacial erosion of sulphide-bearing lithologies. The oxidation of disseminated and comminuted sulphides in these Devensian tills is accelerated by the soil cracking that accompanies the development of summer soil moisture deficits under the lower rainfall conditions of eastern Britain.
