An inlier is an area of older rocks surrounded by younger rocks. Here, you are standing on top of the Bavelaw Inlier comprising rock which formed at the bottom of an ocean about 440 million years ago in the Silurian period. Some 40 million years later, when the “Scottish” and “English” tectonic plates started to collide, these layers of rock were forced upwards, almost to the vertical, to the surface to form mountains. Then later, around 350 million years ago, these rocks became surrounded and mainly overlain by red sandstone, as sand was deposited in the great rivers which dominated this area in the hot and wet conditions of the early Carboniferous. This inlier of Silurian rocks are some of the oldest rocks in the Pentlands Hills and can be seen at the Howe, near here at NT187619.
The junction between the two rock types, one 440 million years old and the other 350 million years old , heavily overgrown, lies to the west of the path from Bavelaw to Loganlea Reservoir at NT175 626. See Photo 2. The adventurous walker, with one foot on the old Silurian and another on the later Carboniferous red sandstone can justly claim to be straddling 100 million years.
In the quarry, see Photo 1, here south of the top of Hare Hill you can see the early Carboniferous red-brown sandstone layers, which are tilted gently to the right. The nearby faulting has hardly affected the sandstone. The older Silurian rocks lie at least 60 metres below the quarry.
Colliding tectonic plates and volcanic action have played an important part in the geology here but the last significant shaping of the Pentland Hills, including the Bavelaw area, took place little more than 15,000 years ago when glaciers overwhelmed the hills.
Questions
1. To what period does the rock on the top of Hare Hill belong?
2. What structures can be seen in rocks in the quarry on Hare Hill, and what do they tell us about the environment when these rocks were formed?
3, Why can we not see the rocks of the inlier, the older grey Silurian rock?
4. What has been the most significant eroding force in the last million years, and responsible for giving us the landscape we see today?
Please now go ahead and log this cache. A photo is always welcome.
For more information about the geology of the Pentland Hills please go to Pentland Hills Regional Park
This geocache has been provided by the Lothian and Borders GeoConservation.