Much of the Pentland Hills is made from igneous rocks, often volcanic ash and lava. At this place, however, we find an exposure of sandstone - a sedimentary rock. Sedimentary rocks are mostly softer than igneous rock, so they tend not to form prominent features of the landscape. However, in their own way the sedimentary rocks are equally as important as the volcanic rocks in understanding the landscape and the geology that dictated its present form.
Layers of sedimentary rocks are called strata. Each different bed is formed in a particular environment that gives it its special appearance. The oldest sediments in this area are these thick sandstones which formed 370 million years ago when the area was barren mountains and deserts. Sand, eroded from even older rocks, was blown by hot winds into valleys scoured by flash floods, often forming short-lived lochs.
This accumulated sand was then compacted by the pressure of more material gathering on top, and then cemented by minerals precipitating within the pore spaces between the sand grains to form sandstone. Tectonic pressures can then bend and tilt the sandstone beds, and faults cut through them.
At this location, almost 50m of sandstone sediments are exposed by the cutting of the link road to the A720 Edinburgh City Bypass.
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This is a busy road, please take care!
Questions:
1. What is the colour of the sandstone in the cutting? Is it uniform, or does it vary? Why do you think this is the case?
2. The beds of sandstone have been tilted since their formation. By looking at the shallower-sloping side of the cutting, in which direction do they slope down, and at approximately what angle?
3. The sandstone was exposed when this link to the A720 Edinburgh City Bypass was built. The slope in the sandstone beds required different engineering solutions on the east and west sides of the road. Please state on which sides each of the following can be seen:
a) a steeper face where the beds dip into the slope
b) whole layers of sandstone have been stripped along bedding planes
c) softer mudstone and siltstone beds have been walled over to prevent erosion.
d) holes have been drilled to drain water from the sandstone.
References:
Lothian Geology: An excursion guide, A. D. McAdam and E. N. K. Clarkson, Edinburgh Geological Society, 1996.
Edinburgh and West Lothian: A Landscape fashioned by Geology, D. McAdam, Scottish Natural Heritage, 2003.