Early Middle Pleistocene drainage in southern central England

Abstract

AbstractThe fluvial sequences of the Milton and the Letchworth formations in the south Midlands of England and neighbouring regions represent at least two pre-existing rivers, the Milton and Brigstock streams, underlying Middle Pleistocene glacial sediments. The Milton Formation includes sand sourced from the Midlands bedrock. This implies that both streams were aligned in a northwest to southeast direction. This direction parallels the contemporaneous courses of the rivers Thames and Trent, the former turning towards the east and northeast to enter the North Sea. Their alignments indicate that the Milton and Letchworth streams formed left-bank tributaries of the Thames, joining the river in Hertfordshire and Essex, as illustrated in the article. This reconstruction has important implications for the interpretation of the proto-Soar river of the south Midlands, represented by the Baginton Formation. Although originally thought to represent a late Middle Pleistocene line, this southwest to northeast aligned system was reinterpreted as the headwaters of a pre-Anglian ‘Bytham river’, a1ligned towards East Anglia. However, recent work has shown that this river could not have existed in the pre-Anglian since there is no link between the Midlands and East Anglian spreads. Recent re-recognition that the Baginton Formation deposits do represent a later, post-Anglian drainage line is reinforced by the identification of the Milton and Letchworth streams, whose catchments occupied the area later drained by the proto-Soar. Overall, the main drainage alignment in southern England during the pre-Anglian period was dominated by northwest–southeast-draining consequent rivers adjusted to the regional geological dip. After widespread drainage disruption caused by the Anglian glaciation, northeast–southwest-orientated subsequent streams eroded frost-susceptible clay bedrock under periglacial and permafrost conditions, and beheaded the courses of some of the older consequent streams.