Some local sandstone beds contain substantial amounts of the mineral feldspar, which decomposes more readily than quartz.    Its presence gives an indication of the geology of the eroding northern landmass from which the rivers carried sediment.     Feldspar is common in igneous granites and metamorphic gneisses, rocks that are still found in the Scottish Highlands.

Magnification of Rough Rock seen through a microscope

Most flaggy sandstones contain flakes of a mineral called muscovite mica, which appears white and shiny and reflects light like a mirror.     Mica breaks up into small bright flakes, which are easily carried in river currents.     In still water, away from the main channels, muscovite flakes were the last to settle on the bottom and coat the bedding planes of flaggy sandstones.

Carbon is also a common mineral in some sandstone beds.     Forests on the delta plain just above sea level, contributed branches, roots and leaves which floated down the rivers.     Large branches and roots caught on sandstone banks are sometimes preserved as fossils in cross-bedded sandstones.     Leaves that drifted into stagnant water decomposed to carbon that was left on the bedding planes to be buried by later sand deposits.     The black flakes in the Rough Rock would have originated in this way.     In the Readycon Dean sandstones there are thin black films of carbon on some bedding planes.     Towards the top of the Millstone Grit series, thin coal seams; several centimetres in thickness become more common.    Some are thick enough to be worked in early pits, for example the Upper Meltham coal.

For more information about the Rocks and Landscapes of Huddersfield see our other pages on Local Geology and our guide to the Geology of Huddersfield. Publications