The Story of Chrome Hill – a reef knoll
Looking up to Chrome Hill from the village of Hollinsclough, it is difficult to imagine that some 350 million years ago, Britain was south of the equator and covered with tropical seas. During this Carboniferous period the clear warm seas were teeming with life such as brachiopods, corals and crinoids. (Crinoids were particularly abundant in the Peak District limestone and are sometimes referred to as 'Derbyshire Screws'). Rainfall was low and the nearest land mass was some distance away to the north, so little sediment was washed into the sea and the water was clear. As the remains of these invertebrates fell to the sea bed they formed a limey mud which became increasingly more solid as limestone.
Although limestone is essentially calcium carbonate plus some impurities, it can exist in a variety of forms; one of the most significant variations is due to reefs where corals flourished, generally on the edge of deep water. In several areas large reefs were formed known as 'reef knolls', such as Chrome Hill and Parkhouse Hill - because of their chemistry and stratification they are more resistant to erosion than normal limestone. After the basic limestone had been laid down, sea water circulating underground precipitated previously dissolved lead, copper, fluorspar and other minerals in spaces caused through faulting and settlement.
At a later period around 300 million years ago, a range of mountains was pushed up to the north and an immense river spread its delta over what is now England, Holland and Belgium. This river dumped sand and mud and buried the limestone. Sorted by the tides and currents, the muds were deposited in deeper water and became shales and the sands became gritstone.
Later, still not far from the equator, the area was covered by swamps which were occasionally flooded by the sea. The plants which fell into the water did not completely rot but they in turn were buried and became coal. The coal layers which covered the Peak have mostly eroded away and only appear around the edges of the area. The main remnant of coal in the Peak is in folds south and west of Buxton and mined in the 19th century.
Between 280 – 60 million years ago Derbyshire was at or below sea level and as Britain slowly drifted north, minerals were deposited in faults and caverns. The Mercian Highlands were formed where Wales, East Anglia and the Midlands now are and immense pressures caused the rocks to be folded. Around 30 - 60 million years ago the rocks of the Peak District were pushed upwards to form a broad fold that is now known as the Derbyshire Dome. When exposed to the ravages of weather the rocks began to erode and the softer and higher the rocks, the more rapid was the erosion. The soft coal was on the top of the pile and the folding pushed it up even higher so it eroded rapidly at the centre of the dome revealing the more resistant Millstone grit. The grit was much thinner in the southern areas of the Peak and it has been eroded revealing the oldest of the rocks – the limestone.
About a million years ago a series of Ice Ages began which were separated by periods with a climate similar to that of today. In early glaciations, the Peak area was covered by ice hundreds of feet thick but the ice front of the last Ice Age came no further south than Leeds. The Peak District underwent a period of intense, dry, cold and as the deeply frozen surface layers thawed, landslips were released and boulders slipped downhill on buried ice layers.
In more recent times, quarrying for limestone and the mining of minerals has had a significant impact on the historical landscape. Calamine was found in Chrome, Parkhouse and Stoop Hills. Research suggests that, between the 1750's to c1810, a few of the local farmers would mine the calamine and it is likely that they did this to supplement their income during the winter. The rocks bearing the calamine would be taken, most probably by packhorse, to be roasted and ground at Milldale. The calamine was then taken to Cheadle where it was used by the Cheadle Brass & Copper Company to make brass. The quantity of the calamine deposit in the three hills was insufficient to make mining a commercially viable proposition.
So, looking at beautiful Chrome Hill which dominates over Hollinsclough, how wonderful to contemplate that the limestone in the Peak District was formed over a period of some 50 million years from the remains of billions and billions of tiny shells and skeletons.