Fph iv. Late interglacial rejuvenation and dissection (late Substage III to Early-glacial)
There is no Holocene analogue for the late interglacial climatic deterioration. Events during this period must therefore be read solely from the geological record of past interglacials. However, fluvial sequences representing the latter parts of interglacials and their transitions into the following cold periods are extremely rare. Whilst this in itself is significant – a point that is addressed below – it is important to begin by considering the events that occurred.
On the basis of the sequences from several sites noted above, it appears that sedimentation continued during the late-temperate phase. But as the climate deteriorated, and presumably mean annual temperatures declined, discharges became more seasonally-variable and perhaps extreme events more frequent. Opening of the woodland, resulting from the decline of mixed deciduous-coniferous forest and its replacement by conifer-dominated woodland (particularly the replacement of fir and spruce forest by post-temperate pine or birch forests), had a marked effect by increasing infiltration. Vegetational retreat and thinning may have progressively increased run-off to the extent that storm-induced flow peaks were raised and water velocities increased. This potentially was accompanied by the increased winter storage of snow on the land surface, as winter temperatures decreased. In turn, this may have initiated gullying on valley-side slopes, stream headwater areas and and locally on some floodplain surfaces, delivering increased volumes of fine sediment to the river. Modern Holocene process analogues (see above) clearly demonstrate that vegetation clearance, initiated in this case by humans rather than by natural processes, result in substantially increased inorganic alluviation. To some extent the late interglacial climatic deterioration would have had a comparable effect, if it were accompanied by more seasonally-variable discharge, as with an early spring snowmelt peak. Higher individual discharges may have occurred, to be marked by thick accumulations of sands and fines on floodplain surfaces as a direct consequence of increased overbank flooding. This flooding would be further exacerbated by the restriction of the river to single, deeper flow-channels, resulting from the prior infill of the final remaining floodplain depressions.
Several sequences record substantial late-interglacial alluviation. For example, at the last interglacial (Ipswichian = Eemian) at Histon Road, Cambridge, fine, inorganic sedimentation begun in the second half of the interglacial (Ip III) and continued into the beginning of the Devensian (=Weichselian), ultimately filling the valley to a depth of over 9 m. That this sediment is preserved results from the subsequent diversion of the River Cam to a valley further to the east leaving the late interglacial channel sediments isolated. A comparable sequence occurs in the Hackney Downs – Stoke Newington area, North London. Likewise at Swanscombe in Kent, the famous River Thames deposits represent an early interglacial event, overlain by cold-climate valley fill sequence. Here rejuvenation of the Thames, late in the Hoxnian (=Holsteinian: Substages Ho III-IV) Stage, is represented by pebbly sand and sand resting on, and in places incised into, the pre-existing early interglacial floodplain and channel sediments beneath. Deposition apparently continued into the subsequent cold stage (Wolstonian or Saalian).
These ‘late interglacial’ sediments are relatively less fossiliferous by comparison to the highly organic floodplain deposits of phases Fph ii or early iii because of the increased inorganic component; reflecting environments which are less favourable to fossil preservation . Moreover where the sediments became desiccated by being situated predominantly above the floodplain watertable, they have become weathered by subaerial processes, particularly oxidation. Decalcification would also have caused carbonate fossils to decay and disappear in extreme situations.
The renewed erosional and sedimentational activity of Fph iv river channels is also likely to have been destroyed prior channel and channel-marginal deposits, especially perhaps of Fph ii/iii age. Higher discharges and sediment loads involving increased channel size and lateral erosion / accretion along the course of previously quiescent channels would probably remove much of the evidence for prior channel activity. Channel metamorphosis from inactive meandering / anastomosing to braiding in particular has opposite effects than where the style transition is the reverse (as in Fph 1 ). The small channels and topographically smoothed floodplains of Fph iii encourage channel transformation (enlargement), overbank coarse splays, and the redevelopment of lateral-accretion styles capable of removing large tracts of previous sediments.
As the climate deterioration into the glacial period, woodland disappeared and was replaced by regional herb-dominated early-glacial grassland. Typical cold-climate regimes with highly-peaked flow discharges ensued. These provided the energy for stream rejuvenation, channel enlargement, remobilization of coarse debris, rapid removal of fines, and substantial incision into accumulated floodplain deposits. Furthermore, gullying of valley-side and floodplain sediments, regolith and soils would lead to the incorporation of reworked fossils, especially pollen and spores, into these accumulations. Just as early interglacial deposits involve reworked glacial-phase sediments, so late interglacial deposits incorporate a mélange of interglacial soil and organic fines. The redevelopment of permafrost further ensured rapid surface water flow as infiltration became inhibited, and with it slope erosion and solifluction exposed regolith and substrate materials providing a source for coarse clastics materials for rivers. The re-activation of conditions for gravel transport, but without the full loading of glacial inputs, resulted in valley incision, removing any remaining fine alluvium en masse. At the same time, the upper parts of floodplain and channel deposits were planed-off by laterally mobile and incising rivers. Previously accumulated fines were transported down-valley as both disaggregated particles and occasionally as coherent blocks (e.g. Somersham, Cambridgeshire) resulting from channel-margin undercutting. It is as yet not clear precisely when gravel transport was re-activated with respect to the interglacial-glacial transition. Inevitably this may vary between different interglacial events.
Clearly the preservation potential of fine-sediment accumulations in otherwise high-energy gravel-bed dominated streams is particularly low, as observed frequently in modern analogue environments, hence the fragmentary nature of the preserved sequences. This fragmentation and constant reworking of the sediments was relatively far greater in situations where rivers were restricted to a narrow valley by steep, often bedrock-controlled slopes, in contrast to where they were unrestricted and able to increase the valley width on non-cohesive or unresistant substrates.