Tunnelling out of a drift filled hollow under Moorgate

Drift Filled Hollows (DFHs) are anomalous, often steep sided depressions in the upper surface of the London Clay or other pre-Quaternary strata. These depressions are usually infilled with Quaternary strata (‘Drift’) or melanges of Quaternary strata and pre-Quaternary strata. The most common infilling materials are sand and gravels generated by Pleistocene glacial outwash to the north of London in the early warming phases between glacial and interglacial cycles. These materials were laid down in successively lower floodplain terraces by the River Thames and its tributaries. These are the ‘River Terrace Deposits’ (RTD). The largest DFHs found to date are several 100m across and 10s of m deep. In some cases DFHs provide a hydraulic connection between the upper and lower aquifers in London.