In the North West Highlands of Scotland, there’s a stretch of coastline graced with the, often understated, natural beauty of creamy soft sands and tempting turquoise water, and bedrock that tells a story far more captivating than man has ever written for himself.
Much of the lowlands here are made of grey metamorphic “Lewisian” gneisses that record some of the earliest parts of Earth’s history available in the rock record. A world so old and foreign that it’s amazing we can tell anything about it at all.
A common feature of the gneisses are textures that tell a story of stretching in conditions deep and hot enough to reach the point at which rock deforms like soft toffee. Harder, darker layers made of minerals rich in iron and magnesium minerals (a.k.a. mafic) respond differently to deformation compared to the lighter layers made of minerals richer in silica (a.k.a. felsic). The contrast in response to extensional deformation result in the breaking up of the darker layers into chains of sausages (aka. boudinage). Each individual block, often lensoidal in shape, is known as a boudin (i.e. sausage).
These boudinaged dark layers in the gneisses are common not just here, but in geologically similar areas around the world, and can be seen on scales of a few millimetres to many tens of meters.
At the beach of Achmelvich (58.172915, -5.302604, [map]), arguably one of Britain’s most stunning beaches, you find gneisses which show clear evidence of the type of deformation mentioned above. There, the gneisses are folded, re-folded, and folded again into a complex, contorted basement which has been the subject of intense unravelling by geologists for over a hundred years.
As you turn you back to the shining blue sea and the clean cream sands, you find a crag with a whole set of these boudinaged dark layers. The brunt of the sea has, over thousands of years, gouged and gouged at this face. The pummelling and battering of the crag has plucked out one of these boudins, a pip of darker rock, leaving a pseudomorph of the former rocky sausage.
The half-egg shaped recession in the cliff is actually a surprisingly perfect perch for the derrière of most beach dwellers or geological guests, and has over the years been used for such a purpose.
But alas, the boudin is nowhere to be seen, and the perch is there like a memorial to the former knocker which ironically got knocked out. Almost in tribute to the boudin, geologists visiting the beach began paying their respects by assuming the form of the boudin for a brief, but immensely symbolic moment. For not only is this a sign of geological respect, homage to the “saus-age”, it has become an important ritual of initiation which marks a milestone in ones geological career. A milestone which will divide ones life into pre- “birth by boudin” (BBB) and post-BBB.
So, should you wonder out yonder, please have a ponder, on the act of birth by a boudin. And when you have, drop me a line and a photo so that we may appreciate your “be the boudin”!