Exhibition of Geology in Hertfordshire, 2 March 2013

Diagrams and fossils from Harpenden

Curated by Les Casey

Harpenden lies in a dry valley in the Upper Chalk dip slope of the Chilterns Escarpment. The top of the chalk hills is overlain by patches of clays, flint pebbles and sands and remnants of Hertfordshire Puddingstone.

Photo: Illustrative image for the 'Exhibition of Geology in Hertfordshire, 2 March 2013' page

When the glacial era was ending melt-water found a dip in the chalk escarpment and flowed down the dip, cutting a valley in the Upper Chalk and laying deposits of glacial gravels – materials carried south by the ice sheet - on the valley floor. Subsequently the river cut the valley deeper so the gravels form terraces on the sides of the Lea Valley.

The valley in which Harpenden village lies was formed by a stream when the post-glacial weather was wetter. Since then the level of saturation in the chalk has declined and the drainage now is by percolation through the permeable chalk beneath the surface. The stream which sometimes ran along the Lower High Street (now in a culvert) to the gravel pits and ponds is just the storm-water run-off from the roads.

Fossils from the Upper Chalk have been found at Harpenden. The best-preserved are those which have been cast in flint. A number of such cast of the heart-shaped sea urchin Micraster have turned up in garden soils along the side of the Lea Valley near the old railway line. Other finds were a different urchin, Echinoccrys, and a sponge, Ventriculites.

Photo: Illustrative image for the 'Exhibition of Geology in Hertfordshire, 2 March 2013' page
Photo: Illustrative image for the 'Exhibition of Geology in Hertfordshire, 2 March 2013' page
Photo: Illustrative image for the 'Exhibition of Geology in Hertfordshire, 2 March 2013' page
This page was added by Rosemary Ross on 07/05/2013.

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