Derbyshire atolls: Parkhouse Hill
Around the margins of the limestone plateau, where shallow-water limestone sedimentation gave way to deeper water shales and muddy limestones, numerous irregular mounds of unstratified reef limestone are developed. Their depositional environment was similar to modern conditions in the south Pacific, with a frame of reefs fringing a shallow tropical sea-floor. Thus we can imagine the Peak District in the early Carboniferous period to resemble a large Pacific atoll of today.
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The reef limestones consist of fine grained lime mud formed by calcareous algae which trapped the limy sediment, causing a build-up on the sea floor of lime-mud mounds (the reefs). Sheltering in the algal framework were many shelly organisms and today the slopes of the reefs contain pockets of beautifully preserved fossils, especially brachiopods and molluscs. The reef mounds frequently form conspicuous features such as the spectacular knobbly forms of Parkhouse Hill and Chrome Hill at Earl Sterndale. The shape of these hills largely represents the original reef topography as they are being exhumed by differential erosion of the relatively soft shale cover rocks.
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