The View from Lepton Edge

Thursday, 10th April 2003, West Yorkshire

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From Lepton Edge, near Huddersfield, you look west towards Castle Hill, three miles away and the saddlebacked ridge of Shooters Hill beyond it. You're looking back in time over Lower Coal Measures rocks laid down some 300 million years ago during the Carboniferous period.

Beyond Shooters Hill, Pule Hill is capped by Pule Hill Grit, which belongs to the Millstone Grit series of rocks. These grittier sandstones dates from the middle of the Carboniferous period, some 320 million years ago.

Outcrops of sandstone and gritstone

Tough layers of sandstone and gritstone cap the hills and moors. The softer shales sandwiched between them have been eroded away to create valleys. It's not quite as simple as that of course because some beds of sandstone can be found in the valley bottoms.

Bread-and-Butter Sandwiches

William Smith (1769-1839), the pioneer geologist who first made the first geological map of England, compared the ridges and vales formed by layers of rock to a plate of bread-and-butter sandwiches.

In this view we're standing on the top crusty layer of a multi-layer club sandwich, looking back over the tilted layers of bread-and-butter (i.e. sandstone-and-shale) below. The Pule Hill grit is the lowest, crustiest, slice of bread we can see.

Pennine Anticline

The reason the Pule Hill Grit is now higher than the Greenmoor Rock, a sandstone that was originally laid on top of it, is that the layers have been tilted to form an upfold, the Pennine Anticline, in a continental collision that took place as the supercontinent Pangaea formed some 280 million years ago. There was further movement along fault lines in the Pennines when the Alps began to form as Africa started to collide with Europe 25 million years ago.

Ice Age Erosion

glacier We might be looking back 320 million years in time but most of the sculpting of this landscape has taken place in the last million years, during at least four advances of ice age glaciers. It's also worth remembering that, because of worldwide changes in climate, for much of the last one million years Britain was warmer than it is today, verging on the tropical. It's thought that tropical weathering might have played a part in the formation of some of the gritstone tors found on the Pennines, for instance at either end of the 'saddle' of Shooters Hill. In warm, wet tropical conditions there might have been deep chemical weathering along the joints and bedding planes of the gritstone.

Ancient Rivers

delta Sand and grit were laid down in river deltas in Carboniferous times. Recent investigations have revealed that different layers of sandstone, for instance the Elland Flags and the Greenmoor Rock, had different origins. Some sandstones have a greenish tinge, others are greyish. Analysis of the heavy metal content reveals that the rivers that supplied the sand originated in different catchment areas.

richard@willowisland.co.uk