Tertiary Rivers: Conclusions
The following conclusions arise from this study:
- The major drainage systems developed at least as early as the land emerged from beneath the sea during the Late Cretaceous and were well-established by the early Palaeocene. Destruction of the extensive Chalk cover followed under a tropical climate with virtually the entire cover being removed over uplifting axes (e.g. the Weald-Artois Anticline, the Channel High). Palaeocene and early Eocene sediments were deposited on an evolving ‘Sub-Palaeogene Surface'.
- Evidence of high-relief erosional episodes is found locally, but rather than an alternation of uplift and relief-reduction phases, it seems that uplift and erosion proceded simultaneously (if episodically) to maintain an overall low-relief landscape in southern and eastern Britain.
- A striking point is the long-term stability of the fluvial system, the major elements being initiated in the late Palaeocene. It is unequivocally demonstrated that the major elements, the Thames, Solent, Irish Sea river and possibly an early Trent existed almost throughout the Cenozoic. This conclusion must be seen against a back-drop of significant, continual crustal deformation and upwarping throughout the period, that continues today. The persistent tectonic regime is the overriding control on the long-term stability of the drainage system.
- The rivers, whilst maintaining their general courses throughout the era, markedly extended and reduced them in response to external changes including tectonic activity and sea-level fluctuations. A comparable pattern is also seen in neighbouring regions of western Europe.
- This constancy is also seen in river form, since meandering streams predominate throughout the period. Although their form certainly varied depending on materials in transport, discharge variability and variations in slope, the overall stability is striking. This appears to relate to the subdued relief, the stability of the regolith resulting from dense forest vegetation cover, and the predominance of fine particulate materials in transport.
- Coarser accumulations dominated by gravels (conglomerates) are extremely rare in the record. Except in local situations, chemically-stable lithologies dominate the clastic component throughout.
- The fluvial responses to sea-level changes appear to parallel closely the reactions seen later on in Pleistocene river systems.
- Low-relief land surfaces dominated the southern British and adjacent regions until the Pleistocene. It is apparent therefore that the deeply-incised river valleys seen today are the product of high, predominantly coarse to very coarse sediment yields, encouraged by the substantial, rapid climate changes that characterise the Pleistocene. This highlights the role of vegetation cover for stabilising the land surface and mitigating flood events. It also emphasises the significance of mechanical (principally frost-weathering) compared with chemical weathering for the rate of landscape dissection and lowering. Moreover, it also demonstrates that climate, and not tectonic uplift, is the primary drive on fluvial incision in the Pleistocene. It is likely that the extensive valley networks (currently largely dry) on permeable lithologies, were also added in this period.
- When viewed from the Tertiary perspective, the Pleistocene evolution represents the most recent ramification of a pattern that has repeated throughout the Cenozoic, albeit modified by glaciation, frost-climate weathering, altered and rapidly-changing climates, and sea-levels. It is reasonable to conclude that the overall pattern of major drainage lines will continue for as long as the current tectonic regime affects the British region.