#mounain

Gibraltar limestone

The famous Rock of Gibraltar has overlooked the entrance to the Mediterranean Sea throughout all of human history. Its dominant position has made it a strategic asset, leading to it being conquered over and over again, most recently by the British, who count it as part of their territory to this day.

The story of the Rock of Gibraltar starts almost 200 million years ago during the Jurassic period. As Pangaea broke apart, there was a huge ocean separating Africa and India from the rest of Asia. That ocean, known as the Tethys Seaway, flooded some portions of the land at the coasts and allowed limestones to be deposited on the shelf. The Rock of Gibraltar is made mostly of the Gibraltar Formation, one of these limestones. It even contains fossils.

About 20 million years ago, Africa was impinging on Europe and beginning to thrust up mountains. The sediments that once sat on the shore of the Tethys Seaway were caught up in this continent collision. They started off as wide, flat-lying beds, but the force of two continents ramming together caused them to fold again and again. The Rock of Gibraltar is part of a specific type of fold called an overturned Anticline.

To understand this kind of fold, I can offer an exercise or a metaphor. Take a stack of paper and lay it flat on your table – this is like the flat-lying limestones that formed in the Jurassic. Now, push the two edges of that paper together and make it fold all the way up; you’ve now created a tight fold called an anticline. Now, tip that fold over to the side, and you have overturned that anticline.

One of the most interesting parts of this fold style is that part of an overturned anticline actually winds up upside down. On one side of the fold, the youngest rocks are on the top, but on the other side of the fold, there are older rocks (at the core of the anticline) sitting on top of younger rocks. This is the case at the Rock of Gibraltar; the upper limb of the fold has totally eroded away, leaving behind only the overturned limb. At the rock of Gibraltar, the older rocks are actually on top and the younger rocks are on the bottom.

There is a weak, poorly lithified, shale unit that sits in this sequence above the Rock of Gibraltar. However, with the overturned limb, the limestone sits on top of that shale. Where the shale is exposed, it erodes easily, undercutting the limestone. The limestone cliff breaks off when the shale holding it in place erodes, leaving behind steep cliffs and skree slopes.

-JBB

-far over the misty mountains cold, to dungeons deep and caverns old-

© feralraven

main tumblr: @ravenbirth