Environmental Engineering Reference
In-Depth Information
Figure 1.2
Environmental and human controls on the
Upper Wharfedale environmental system.
both underground and on the surface (Waltham
et al
. 1997).
Overlying the Carboniferous Limestone Series are about 230 m of strata of the
Wensleydale Group (previously the Yoredale Series) consisting of cyclical deposits of
limestones, sandstones and shales. In turn the Yoredale strata are overlain by about 40 m
of Millstone Grit, a series of coarse sandstones which give the highest flat-topped peaks
in the Yorkshire Dales, as on the eastern side of Upper Wharfedale at Buckden Pike (702
m) and Great Whernside (704 m).
In terms of physiographic evolution, the most influential events were the many glacial
episodes during the
Pleistocene
epoch of the
Quaternary
period. During the most recent
glaciation, namely the
Devensian
, glaciers entered Upper Wharfedale from local ice
accumulation centres on plateau summits in the Pennine uplands such as Langstrothdale
Chase, and flowed south-eastwards down Langstrothdale. On meeting Buckden Pike, the
ice split into a stream flowing north-eastwards along Bishopdale and into Wensleydale,
and a stream flowing south-eastwards down Wharfedale. The Wharfedale ice was joined
by Littondale ice, which together eroded the truncated spur of Kilnsey Crag in Great Scar
Limestone. The power of glacial erosion is evidenced by the classic U-shaped valleys of
Upper Wharfedale, Littondale and Bishopdale.
During periods of glacial retreat, moraines were deposited in valley bottoms and on
lower valley side slopes. These are thought by some geographers to have initially
dammed the river flow, but to have been breached since. According to this school of
thought, the present flat alluvial floor of Upper Wharfedale held lakes in late glacial and
early postglacial times. During deglaciation, when subsurface drainage was prevented by
permafrost
, meltwaters eroded marginal channels along valley sides. At Conistone
village the impressive ravine of Conistone Dib narrows to 1 m in width in places, with
fluvial potholes evident on the side walls. This dramatic feature was eroded by a
glaciofluvial stream, either from the front of or beneath a retreating glacier.
Since the Pleistocene epoch, geomorphic activity in Upper Wharfedale has taken two
main forms. First, the weathering and erosion of hillslopes have produced valley-side
scars and small screes. Secondly, the main channels of the river Wharfe and its tributaries
have been carrying out fluvial action, with periods of incision (i.e. vertical erosion)
alternating with periods of lateral channel migration and aggradation (i.e. deposition),
