Palaeogene conglomerates (puddingstones) in the Colliers End outlier, East Hertfordshire, UK – evidence for age

The siliceous conglomerate called Hertfordshire Puddingstone is rare in situ, hindering the dating of its formation; most finds are made in fields or stream beds. The Colliers End outlier on the northern rim of the London basin yields many finds of puddingstone with a fine grained pale matrix containing rounded brown and grey coated, internally stained flint pebbles and some cobbles. The Palaeogene succession of the outlier includes Upnor (resting unconformably on the Chalk), Woolwich/Reading, Harwich and London Clay Formations. There are pebble beds at the base of the Upnor Formation and between the Upnor and Reading Formations. Large quantities of loose ‘puddingstone pebbles’ (flint pebbles which appear identical to those in concretions), some with patches of matrix adhering to them, occur on surfaces around +89 m to +90 m OD, the height of the boundary between the Reading and Upnor Formations. Pits in one such area are interpreted as dug by Romans quarrying puddingstone (Lovell and Tubb, 2006 and Green, 2016).The level of the loose ‘puddingstone pebbles’, borehole records, data from road widening and the Roman quarry suggest that a layer of pebbles were deposited on top of the Upnor Formation, then stained, mixed with fine white sand and patchily cemented early in Reading Formation times during a depositional hiatus in a seasonally dry sub-tropical climate. Further work is needed into (a) the mechanism which stained the pebbles, (b) why silicification was patchy and (c) the stratigraphic position of many large puddingstones removed during construction of the A10 (Colliers End bypass).