.Superficial Deposits.

Compared with the solid formations, the superficial or Quaternary deposits of Hertfordshire are much more variable in nature and are often laterally impersistent. They originated in various ways, the four most extensive types being:

1. Gravel deposits of the River Thames, dating from a period when it flowed north-eastwards through the Vale of St. Albans rather than following its present course through London.

2. Clays and gravels deposited by a glacier, which entered NE Hertfordshire about 400,000 years ago (the Anglian Ice Sheet – see map above) and blocked the earlier course of the Thames, thus causing the southward diversion through London.

3. Sediments deposited by the wind under very cold dry conditions or sludged down slopes such as valley sides when the surface layer of a frozen soil melted in summer sunshine.

4. The Clay-with-flints, which forms a thin layer (<15 ft) over the White Chalk on the Chiltern Hills.

During the last 2 million years or so, the climate of Britain changed many times, from a moist temperate environment rather like the present day to very cold and often quite dry arctic conditions. The cold periods are often termed glaciations, because they resulted in the development of glaciers, often covering much of northern Britain, and the warmer periods are known as interglacials. Types (1) to (3) above were deposited in one or more of the most recent cold periods. Interglacial deposits were mainly formed in small lakes or streams, and are therefore even more localised in occurrence.

The Gravels deposited by the Thames in cold periods form a series of gently sloping river terraces along the earlier course of the river through the Vale of St. Albans. The oldest are at fairly high levels on the Chilterns, and younger terraces occur at successively lower levels. Because of subsequent erosion each terrace occurs as patchy remnants. All are now composed mainly of angular, frost-shattered flint fragments, but originally they would also have contained large amounts of angular chalk fragments, which have been lost by prolonged weathering.

The grey Chalky Boulder Clay resulted from the Anglian glaciation (see map above). Both the clay and associated gravels contain fragments of chalk, flint and many other rocks brought by the glacier from Northern England, Scotland and even Scandinavia. The glacier reached as far south-west as Bricket Wood, and beyond this the water of the Thames was impounded to form a large lake. Southward overflow from the lake formed a new course, which developed into the present Thames valley through Middlesex and central London.

Loess (formed of silt) was deposited by wind during the later cold stages of the Quaternary, covering the whole county, forming thin layers (usually 1-3 ft) over the Thames terraces, Anglian glacial deposits and Clay-with-flints. Thicker accumulations are present in many valleys, such as that of the River Lea south of Ware. The surface horizons of soils almost throughout the county are formed from this layer. It makes them fertile and very suitable for agriculture because the loess is easier to cultivate and retains more plant-available water than the underlying gravels or stony clays.

The Clay-with-flints occurs on higher parts of the Chilterns above the level of the highest Thames terrace. It often contains blocks of Hertfordshire Puddingstone and its mineral composition is like that of the Reading Beds. It is therefore thought to have originated by frost-disturbance and interglacial soil development in a thin veneer of basal Reading Beds surrounding the Reading Beds outliers.

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