Half a mile up the track that runs past our house there's a small quarry, mostly mined for its rock to be crushed down as aggregate for use in maintaining the estate roads. A closer look at the rock shows that....
....it is formed of particles of previous rocks varying in size from cobbles down to finely-crushed grains. Normally, if the larger particles are fairly rounded, the rock is termed a conglomerate; if the particles are angular, it's a breccia.
An even closer look shows that there is considerable variety in the constituent rocks. This would normally suggest that, whatever mechanism(s) produced the rock derived it from an extensive area. So, for example, the rock might have been formed as a beach, with the materials brought from miles away by the sea or by nearby rivers.
My knowledge of geology is rusty from the years of neglect in favour of other interests such as archaeology, but I feel that this rock was formed rapidly, but not so rapidly that the agent moving the material didn't have time to round some of the larger particles and sort the different grades. So the agent of erosion might have been a river, though a river might have sorted the particles more than is seen here.
What I do feel about this rock is that it was formed quickly and violently.
That is one of the most beautiful and diverse metamorphosed sedementary rocks I have ever seen. Such a number of rock types and degrees of roundedness - there are some quite angular ones in there too. Is it contemperaneous with the Old Red Sandstone? If so, it probably wasn't anything to do with glacial erosion due to equitorial positioning. I understand there were a lot of big rivers and fast errosion of the Caledonian mountains of Laurussia at that time. I think that rock deserves a position in your garden if you can negotiate ownership (and delivery)!
ReplyDeleteWhile the rock hasn't been metamorphosed, it is part of the Devonian Old Red Sandstone. The mixing of angular and rounded fragments, and the presence of so much matrix between the cobbles, takes a lot of explaining. I know that some of the local ORS beds were formed when a fault moved vertically, which would have produced a scarp face down which previously collected sediments would have collapsed.
DeleteI love the imaginative side of geology.
Jon
Oh, and we do have some big boulders of this rock in our garden!
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