Some time back I shared my photos of Hutton's famous unconformity at Siccar Point, Scotland. Yesterday Chris over at Highly Allochthonous shared a video he made which gives one a much better view of the unconformity than still photos. Thanks Chris!
I enjoyed another visit to Scotland last week. While there I went up to St. Andrew's, home of the third-oldest English-speaking University in the world. In addition to a noteworthy University, the town's other tourist attractions include the ruins of a castle and a cathedral.
These first two photos show what is left of the castle up at the ground level.
This is the cliff just to the left of the castle, showing the sedimentary rock upon which the castle is built.
This is a tunnel under the castle. During a siege the attackers dug a mine, hoping to get under the walls and blow them up. But the folks in the castle heard them digging, dug a counter mine, and managed to fend off the attackers in the tunnel.
Here is a view through the walls down onto the beach, showing more of the sedimentary rocks below.
While the rocks in this part of Scotland aren't as pretty as the metamorphic rocks in the interior of the country, still it was fun to see the interaction of the human history and the geology.
The conference started with a local field trip. We went first to Hollyrood Park, which is in the middle of the city. The most prominent feature of this park is Arthur’s Seat, which is the core of an extinct volcano, which erupted about 350 million years ago. Ten distinct lava flows (olivine +/- other minerals basalt) are distinguished in this area. Apparently there has not yet been a systematic flow-by-flow detailed comparison of the various compositions, if any students out there are interested in the subtle changes that a volcano undergoes during its eruptive cycle.
The second most prominent feature of the park is the sill which makes up Sailsbury crags. James Hutton, (a local man who is known as the father of geology) found an important contact here which helped to settle the debate between the “Neptunists” and the “Plutonists”. The former believed that crystalline rocks formed by precipitating out of sea water, while the latter believed that they had cooled from a molten source.
This outcrop showed that the crystalline rock had intruded into sedimentary layers and in the process, lifted up a layer of sandstone and oozed a short distance underneath that layer. This would not have been possible had it precipitated out of a liquid. Additional proof was provided by his friend, Sir James Hall, the father of experimental geology. Hall owned a foundry, so had access to the appropriate equipment to try melting rock. His experiments proved that one could melt basalt, cool it, and obtain a rock which looked much the same as the starting product. Therefore, the original must also have cooled from a molten source.
Another noteworthy feature of the park is the world’s first example of geological conservation. Hutton observed this hydrothermal vein cutting through the sill, altering the minerals and depositing hematite (iron ore). At the time this area was being actively quarried, and he requested that the quarrymen leave this particular part of the outcrop intact for future generations to observe the phenomena, and they did. The cliff wall is now quite a way behind the preserved portion of the outcrop.
Views from the park include Edinburgh’s castle, which sits upon another volcanic plug.
Stay tuned for my next post, when the field trip moves about 40 miles south to Siccard Point, the site of Hutton’s most famous unconformity.
Posts
All Comments
