Quaternary history of the Ayot Paleogene Outlier,Southeast England: A field and laboratory case-study in local geology with regional implications

Despite limited exposures, detailed field mapping and sedimentological analyses – particle size, clast petrography and fine-sand mineralogy (plus limited coarse silt and clay mineralogy) – have elucidated the ages, provenance and depositional environments of the complex and controversial Quaternary deposits covering the Ayot Paleogene Outlier. Most of the sediments were largely derived directly and/or indirectly from local facies of the Woolwich and Reading Formation. The oldest deposits are (Pre-)Pastonian proto-Thames gravels overlain by thin sands, which collectively represent crucial evidence for the earliest (Stoke Row) terrace of the proto-Thames. These materials were later thrust southwards by the Anglian ice close to its margin, which deposited a thin yet heterogeneous till at Ayot. This unit is correlated with the Ware Till, suggesting that the Anglian ice was thicker during the earlier of its two major southwestward incursions into the Vale of St Albans. An aeolian mantle was deposited on the Outlier during the Late Devensian; most of the blown material was incorporated into the upper regions of the till but some accumulated in depressions to form brickearths that resemble some Chiltern Brickearths. A colluvial apron developed during the Holocene, together with three swallow-holes. Some of these depositional events were separated by periods of poorly-preserved pedogenesis.