Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

agricultural land lies on valley-bottom deposits of reworked soil eroded off
surrounding slopes.
Neolithic settlements in southern France are concentrated almost exclusively
on limestone plateaus known today for bare white slopes sporting
thin, rocky soil and sparse vegetation. When farmers arrived, these uplands
were covered by thick brown soil that was far easier to plow than clay-rich
valley bottoms. No longer suitable for cultivation, and considered something
of a backwater, the limestone plateaus around Montpellier are used
primarily for grazing. The harbor at nearby Marseille began filling with
sediment soon after Greek colonists founded the city in 600 bc. Sedimentation
in the harbor increased thirtyfold after agricultural clearing spread
up the steep slopes around the new town.
Early forest clearing in Britain led to extensive soil erosion long before
the Roman invasion, as a growing population slowly cleared the forest to
plow the slopes. High population density in Roman times exacerbated the
loss, in part because better plows worked more of the landscape more
often. The population fell dramatically as the empire collapsed, and took
almost a thousand years to build back to the same level.
Floodplain sediments along Ripple Brook, a small tributary of the River
Severn typical of lowland Britain, record a dramatic increase in deposition
rates (and therefore hillslope soil erosion) in the late Bronze Age and early
Iron Age. The relative abundance of tree pollen recovered from valley bottom
sediments shows that between 2,900 and 2,500 years ago the heavily
forested landscape was cleared and intensively farmed. A fivefold increase
in floodplain sedimentation speaks to a dramatic increase in hillslope
erosion.
Net soil loss averages between three and six inches since woodland clearance
in England and Wales. Some watersheds have lost up to eight inches
of topsoil. Although much of the loss occurred in the Bronze Age or
Roman times, in some places substantial erosion occurred after medieval
times. Just two hundred years after Nottinghamshire’s famed Sherwood
Forest was cleared for agriculture, the original forest soil has been reduced
to a layer of thin brown sand over rock. Just as in Lebanon’s ancient cedar
forest, most of the topsoil is now gone from Robin Hood’s woods.
Across the border in Scotland, radiocarbon dating of a sediment core
recovered from a small lake west of Aberdeen provides a continuous record
of erosion from the surrounding slopes for the past ten thousand years.
Sediment deposition rates in the lake, and thus erosion rates on the surrounding
slopes, were low for five thousand years under postglacial shrub-
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