Case Studies:Case study 20
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[edit] Evidence for Medieval soil erosion in the South Hams region of Devon, UK
Foster et al.’s (2000) paper researches the impact of land-use changes on alluvial and colluvial deposition using a variety of palaeoenvironmental techniques (pollen, charcoal, mineral magnetism, sediment chemistry). The chronology of alluvial fans and debris cone sedimentation in upland environments in the British Isles show widespread evidence for phases of increased sedimentation rates during the tenth to twelfth centuries and the sixteenth to nineteenth centuries. This may have been due to climate and land-use change.
The research site is located in the South Hams region of South Devon. Stratigraphical descriptions, coring and sample collection has been undertaken within the Slapton Sewage Works marsh. Samples for physical, geochemical and mineral magnetic analysis were recovered by using 5cm diameter plastic drainpipe hammered vertically into the sediments and retrieved using a tripod-mounted chain hoist. The plastic drainpipe core of c. 1.5 m length was split in the laboratory and sampled at 1 cm intervals. Each sample was weighed for the determination of wet bulk density and was subsequently oven-dried at 40ºC overnight. Samples were reweighed for the determination of dry bulk density and subdivided for analysis of organic matter content by loss-on-ignition, particle size, mineral magnetic and geochemical properties.
Three possible explanations exist for the deposition of a silty clay layer: a change in the hydrological regime, or in the level of the outlet of the Ley to the sea, resulted in a temporary increase in water level which induced sedimentation higher up valley. Alternatively, the sediment could have been deposited by a river meandering across the Slapton marsh valley. Finally, the sediment could have been derived from within the catchment resulting from an increase in sediment delivery.
Little evidence is present for an increase in water level or deposition within, or at the margins, of a meandering river. Therefore, the most likely explanation for the deposition of this silty-clay layer relates to an increase in the delivery of sediment from the contributing catchment. Two fining-upwards sequences between might be commensurate with rapid influxes of minerogenic sediment over a relatively short period of time (single events to years) although the high organic matter content of the entire deposit might suggest slower sedimentation rates (decades to centuries). The magnetic characteristics and pollen data provide evidence to support the hypothesis that the silty-clay layer can be traced to catchment soils.
The full article is Foster, I.D.L., T.M. Mighall, C. Wotton, P.N. Owens and D. E. Walling (2000) Evidence for Mediaeval soil erosion in the South Hams region of Devon, UK The Holocene 10 (2) 261-271. This can be found at http://hol.sagepub.com/cgi/reprint/10/2/261
Keywords: Soil erosion, climate change, agriculture, mineral magnetism, Medieval period.
