Environmental Engineering Reference
In-Depth Information
result was a sequence of horizontally bedded limestones, overlain progressively by
mudstones and then coarse grits, described in Chapter 1, which dominates the Dales
landscape today (Figure 1 and see Colour Plate 9 between pp. 272 and 273). The
sequence once concluded with coal seams derived from the equatorial swamp forests but
subsequent erosion restricts their modern outcrop to the Pennine margins. Final closure of
the Rheic Ocean came as Gondwana collided with Laurussia to form the
Variscan
orogen
and supercontinent Pangaea
c
. 290 Ma ago. South-west Britain lay on the northern edge
of the Variscan suture zone, whose remnants include the North American southern
Appalachian mountains and Eurasian Urals today.
Figure 1
Geology and related geomorphology of the Ingleborough
massif, western Yorkshire Dales. Tilted Lower Palaeozoic rocks of
the Askrigg extensional fault block form the unconformable
basement to Carboniferous shallow shelf carbonates (Dinantian
Great Scar Limestone) and overlying deltaic sediments (Namurian
silts, sands and grits).
These ancient terranes lie exposed at the modern land surface only after considerable
erosion since formation. Pebbles of Shap granite in the basal Carboniferous Limestone
show that 7-10 km of overlying rocks were stripped off in 60 Ma after its subsurface
emplacement. Although only roots of the Caledonian and Variscan tectonic structures
remain, they still have a profound landscape effect. Quaternary glaciation most recently
etched the Lakes and Dales region. Glaciers scoured a radial pattern of deep troughs out
of the Lake District dome and thinner ice streams moulded the Dales, abrading the
limestone
pavements
and depositing hundreds of
drumlins
in their path. Their traces are
clearly visible on the satellite image, ornamenting the far older tectonic terranes.
