The geology dates back to the Precambrian, and during at least part of that long period the feature would have been around 60° south of the equator, the same as the current latitude of the Falkland Islands. Shropshire would have been at the very edge of a large continent near the sea, which was being buckled by tectonic activity, causing volcanoes to form. The area had broad rivers; evidence of mudflats has been found. The rivers would have flowed out to sea, creating large estuaries; over time, the mudflats would have built up, and volcanic eruptions deposited ash in layers between the sand and mud. The primary rock of the Long Mynd is sandstone, usually coloured purple or grey. The volcanoes created the nearby Stretton Hills and the Wrekin, and eruptions would have been frequent. There are layers in the rocks of the Long Mynd that were previously described as raindrop marks. Unpublished research, including electron micrographs, by the now deceased Professor Martin Brasier, showed that these are actually ichnofossils created by an unknown Ediacaran biota. Field observations of the stratigraphy present, and its laminar nature, leave no doubt that they are in a marine environment; the absence of infilled or mineralised syncresis (shrinkage cracks) adds to this. Examples of these fossilised marks can be viewed today in the National Trust Tearoom Exhibition, in Carding Mill Valley. The layers of rock built up over the millennia to create an approximately thick layer composed of sand, mud, silt and very occasional thin ash bands. The stratigraphy, mineral compositions and surrounding volcanology suggests an infilling island arc basin.