I have this photo as a background image and I often wonder how such a narrow, well-defined slit could be formed. Is it natural or man-made? If natural, what processes could have formed it? The rest of the coastline is rugged but this appears very uniform. It is on the north-west coast of Jersey (UK).
1$\begingroup$ That coast has many more similar features, which suggest it is natural firstname.lastname@example.org,-2.2423271,1287m/data=!3m1!1e3?hl=en $\endgroup$– Jan DoggenMar 16, 2016 at 8:36
If you see the other user (Jan Doggen's) google maps link, you can see apparently similar features in different states - this is a particularly neat example of a general phenomenon.
These notches form when headlands are undercut by caves carved out by the sea. They may be initiated where there is a fault or jointing in the rock (northwest Jersey is granite, according to google).
In this particular case, the notch seems to have a rounded termination. Blowholes also form from undercutting of headlands producing caves whose roofs then collapse. It's possible this cave may have collapsed after a blowhole formed, so you get a notch where the inland end of the feature appears rounded. That bit is just speculation though.
I found this Jersey geology trail guide that might have more detail: http://jerseygeologytrail.net/Geomorph.shtml
Could be formed by two natural processes. First, some fault with shear zone, which is obviously much weaker than the rock around it and gots weathered faster. Second, could be some vein/dyke of some mineral or rock softer than surrounding rocks.