Tuesday, October 04, 2016
A few posts back, I mentioned that Clew Bay, in County Mayo, has partially submerged a large field of drumlins. Drumlins are teardrop shape hills of glacial drift - formed under the ice and indicating its direction of movement. They can be seen best from above (Google view of Clew Bay).
What makes these interesting from a coastal geomorphic perspective is that the waves approaching from the western end of the bay have eroded the leading edge of the drumlins, redistributed the coarse glacial sediment, and built barrier beaches. As these beaches grow, they shelter the islands that are farther east. The evolution of the beaches is complicated. Sediment sources can be depleted as individual drumlins are eroded away. Transport pathways for beach sediment can be created, or lost, as beaches form or break up.
Other examples of coastal drumlins come from Nova Scotia and Boston Harbor. These sites have all led to some classic literature on the formation and stability of gravel barriers. We have drumlins in Puget Sound, too, although they aren't as geologically distinct and certainly not as well researched as elsewhere.
Bertra Beach (also Bartraw Beach) is located on the south shore of Clew Bay and consists of a gravel barrier that connects the mainland with a small drumlin island offshore.
Bertra is located just below one of Ireland's most well known and distinctive peaks - Croagh Patrick - which on the last Sunday of July attracts tens of thousands of pilgrims for the hike to the top.
Footnote: As best as I can tell, the two eroded drumlins in the lead photo are the north and south parts of Dorinish Island, once owned by John Lennon and Yoko Ono. There's a nice photo of the tombolo between them, and some of the back story, in this 2012 Bloomberg article.