Fph i. Late-glacial – interglacial transition (Substages late-glacial to I)
The abruptness of the climatic amelioration that initiated the change from glacial to interglacial (or interstadial) conditions caused mean annual temperatures to increase over a few years. Following an inevitable ‘lag’ of unknown duration (over 500 y in the earliest Holocene), the initial response of rivers to this shift is likely to have been a decline in seasonal flow variability. As result of permafrost melt and a decline in winter severity, water from precipitation and infiltration provided groundwater sources for flow all year around. This is of considerable significance in lowland Britain where bedrock aquifers are common. Stabilisation of gravelly channels inherited from the immediately preceding cold phase occurred (Fig 4a), since rivers broadly had insufficient energy to transport gravel and alter gross channel form and distribution. This concept of inheritance of channel pattern has also been noted consistently in the early Holocene by Brown and Gibbard. Unlike on the neighbouring Continent, incision has not been reported at this transition in the British lowlands.
Initially the decrease and disappearance of the annual snowmelt flood-peak caused a reduction in mean flow velocities and water depth. Thus large areas of the emergent braidplain-floodplain surfaces became free from flood incursions for long periods. This, and the rapid inward migration and expansion of interglacial vegetation, provided a complex of surfaces for plant colonisation and soil development. Vertical accretion of fine-grained, often organic, fossiliferous sediments began in braidplain depressions and abandoned channels. The increased plant productivity and vegetation of floodplain surfaces also encouraged fine-sediment accumulation, by stabilising floodplain surfaces and restricting streams to the courses inherited from the immediately preceding cold stage.
Initially therefore fine-grained sand or silt was deposited inactively or partially active channels, possibly together with some dispersed plant fragments, wood and terrestrial and aquatic molluscs and vertebrate remains. In closed channels, cut-off from flowing water, silts or clays accumulated depending on local source materials. Holocene analogues suggest that this occurred rapidly. Where flooding affected the channels, sand beds or laminae occur decreasing upwards in response toprogressively decreasing accommodation space. In isolated standing-water filled depressions or channels organic sediment predominated. Tufa or marl, often rich in calcareous fossils was deposited by incoming springs.. Eutrophication of pools by accumulation of plant debris produced a transition to black humus-rich detrital sediment, followed by peats as the depressions become infilled.