Fph ii. Full interglacial (Substage II)
With the establishment of fully-forested catchment vegetation cover, river flow variability became diminished further as the soil-vegetation system stabilised ground surfaces and delayed flood response. Stabilisation caused severe reduction in the supply of inorganic sediment to alluvial systems. Streams continued to flow relatively slowly all year round, except following exceptionally severe storms, flowed. Channels therefore stabilised, and minor deposition predominated, forming fine-grained deposits very rich in plant-detritus in the minor channels still open. The high biotic productivity, dense vegetation cover and lack of channel erosion combined to yield only organic detritus. Floodplain vertical accretion dominated the entire system with infill of remaining inherited depressions and pools by detrital or marsh sediment changing to in situ peat growth in the moist floodplain woodland environments. By this phase the channels had cohesive banks. As a direct consequence, watertable levels rose in the floodplains.
In the Holocene this was a phase during which recorded sedimentation was markedly reduced (see above), yet it is very well represented in the Ipswichian and Cromerian interglacials. Attributed to milder climate during the climatic optimum of the Holocene (an episode that may not have occurred in previous interglacials), it may reflect very low sediment production during high biotic productivity, or subsequent sediment removal.
Inorganic sedimentation probably only occurred following storm floods or beaver-dam bursts, such as the ‘diamicton-like’ sediment reported from Woolpack Farm, Cambridgeshire, or where large vertebrates gathered to drink; their activities leading to the destruction of floodplain woodland and the establishment of grass- or herb-dominated clearings. Bank destruction and the uprooting of trees, phenomena commonly associated with large mammals, locally supplied inorganic material to river channels. In addition, large mammals modified sediments locally by defaecation in the water and bioturbation disturbance during walking, bathing and related activities. Such effects have been identified in Ipswichian-age sediments, e.g. at Barrington, Cambridgeshire and in the Cromerian stratotype Upper Freshwater Bed at West Runton, Norfolk.