Holocene fluvial and marine influences and settlement interactions in the lower Ribble Valley, Lancashire, U.K

摘要

The evolution of the lower course and estuary of the river Ribble (Lancashire, U.K.) during the Holocene is the object of study, along with the history of sediment fluxes in them and their influence on past human settlements. Investigation of the valley floor geomorphology and terrace fill stratigraphy and sedimentology, as well as palaeoecological analysis and a number of 14C essays, allowed the reconstruction of Late Pleistocene and Holocene alluvial history and chronology of a reach at the transition point between fluvial and estuarine influences; the archaeological evidence is evaluated in this context. Alluviation and incision cycles led to the formation of four river terraces. The oldest terrace, rich in coarse-grained materials, seems to be of Pleistocene age. 14C dating on the second terrace would point to an Earliest Holocene or Younger Dryas age; the fill, however, comprises abundant fine-grained overbank sediments. Large parts of the unit were reworked during a phase of lateral channel activity that occurred prior to ca. 8900 cal BP; between then and ca. 6900 cal BP the river underwent meander cut-offs, after which limited lateral activity occurred. New alluviation occurred around 4700 cal BP, possibly related to the aggradation of the third terrace, though its fill is only certainly known, from archaeological evidence, to be of pre-Roman age. Channel size increase occurred by the time the terrace was deposited. The fill features abundant clayey sediments; it was incised at or after the end of the Roman period. The last terrace was deposited in the late first millennium AD mainly as overbank silts, and has since been incised again. Fluvial response appears mainly related to climate changes, river activity corresponding to shifts to wetter, cooler climate. Human action likely enhanced fluvial response in the historical period, leading to post-Roman incision and deposition. There seems to be a good connection between sediment production in the catchment and deposition downstream. Local factors also show a major importance in determining river response. Early Holocene fluvial history finds no match in other Northern English rivers; a different response to the same climate changes is apparent, possibly related to differences in sediment supply conditions. No clear evidence is found supporting an influence of sea-level change on river processes. On the other hand, it appears plausible river sediment input has a driving influence in enhancing or causing minor regressions recorded on the estuary. The conditions existing in Roman times could have allowed access from the sea to the Roman sites at the upper end of the Ribble estuary and by its North shore; a relative sea-level minimum could have caused a shift of focus in naval traffic from the former to the latter site.