The Story of Potters Bar and South Mimms. Published in 1966. The Geology and Geography of the District '‘The ground itself, where we can read the information it affords is, whether in the account of the Conquest or in that of the settlement of Britain, the fullest and the most certain of documents.” (J. R. Green, The Making of England. 1881.) MANS environment has always had a profound effect upon his life: in primitive conditions his settlements and occupations were alike influenced or even dictated by the shape of the land, the nature of the soil, and its natural vegetation and resources. The Potters Bar district is no exception, for these factors can explain a good deal of our local history. THE MAIN ROCKS Our district lies near the northern edge of the London Basin, a shallow saucer-shaped depression between the Chilterns and the North Downs, which are its up-turned rims. Beneath the familiar clay which comprises nearly all the surface rock lies a thick layer of chalk laid down in the Cretaceous period, over 70 million years ago, when most of England was a moderately deep sea. As Tennyson aptly wrote: There, where the long street roars hath been The stillness of the central sea. This Chalk is exposed near the Barnet By-pass and in the floor of the valley hewn by the Mimmshall Brook where, resting on it, can also be seen a narrow outcrop of the Reading Beds. These early deposits of the Tertiary period are from 30 to 50 feet thick and consist of pale-coloured sands and beds of flint pebbles, locally cemented together to form the famous 'Hertfordshire Puddingstone. These deposits were laid down by rivers long after the Chalk had been raised above sea level and partly worn away by rain and rivers, and yield no sea fossils but only occasional plant remains. Page 1 of 107 Above the Reading Beds (which give a naturally well- drained subsoil, like the Chalk), lies the London Clay, the chief Eocene deposit of the London Basin. This deposit dates from the early Tertiary period some 50 to 60 million years ago, and forms the bed rock of most of the district. The full thickness of the clay is not preserved here, but 150 feet is recorded in local wells and 200 feet near Shenley. The London Clay is blue-grey in colour when unweathered, but in surface exposures changes to brown. Layers of septaria, cement stones veined with crystallised calcite, occur in it in places. This inhospitable London Clay is similar to the brown or blue mud which forms today in depths of 100 fathoms or more. The clay has yielded few fossils in our area but elsewhere many plant remains and a large fauna, including mammals, birds, crocodiles, turtles, fishes and marine invertebrates, have been found, and indicate a warmer climate than at present. After the formation of the London Clay the land was affected by the world-wide Alpine movements which created the highest mountain chains that exist today. Mild ripples from this great disturbance reached Britain, where the rocks were gently ruckled and tilted. It is one of these minor folds that is responsible for bringing the Chalk to the surface in the valley of the Mimms- hall Brook. Since Tertiary times the physical landscape of Britain has been produced by successive cycles of erosion. These have been responsible for the removal of any later Tertiary materials which might have been deposited here. In thinking of this process we may fitly continue our Tennysonian quotation: The hills are shadows and they flow From form to form and nothing stands, They melt like mist, the solid lands, Like clouds they shape themselves and go. THE OVERLYING DRIFTS The solid rocks—chalk, clay and bedded sands—make up the mass of the land. On them rest certain thin deposits of much more recent date, laid down before, during, and after the Ice Ages of the last million years, and having an important bearing on the mineral resources and land utilisation of the district. It was during this period that the rocks were carved into the hills and valleys we know. There are three main 'drift’ deposits, at different levels. Strangely, the highest is the oldest and the lowest the most recent. First came a deposit of sandy shingle known as the 'Pebble Gravel’, which, as fortunate gardeners know, caps the highest ground in the locality, at about 400 feet. From Barnet this Pebble Gravel covers a narrow ridge to Bentley Heath, and after a short gap continues in a wider sheet along Potters Bar High Street to Little Heath. Other patches exist at Mimmshall Wood, and at a lower level at Dugdale Hill and Dyrham Park. At all these places pits have been dug to obtain gravel. These pits are shallow, for the Pebble Gravel is rarely more than ten feet thick and usually less. It consists largely of rounded flint pebbles such as occur in the Reading Beds, mixed with smaller white quartz pebbles and a few other stones—of which the most interesting are pieces of Lower Greensand chert derived from the Weald and transported by ancient rivers right across the London Basin from the Surrey area. This Pebble Gravel dates from the early Pleistocene period, early in the great Ice Age, and was probably a river deposit brought down by a tributary of the Thames when this river ran well north of its present course, roughly from Maidenhead along the line of the Vale of St. Albans, past Hertford and through Essex. The present St. Albans Vale is, clearly, 200 feet or more lower than at that time, and, more strikingly, the whole of the Middlesex plain and the present lower Thames valley have been carved out since then. Next in order of date are the deposits directly associated with the presence of ice in the region. These materials are part of the complex sheet of the Chalky Boulder Clay which extends into the area from the north and north-east. On top of the plateau, approximately from the Mutton Lane-Barnet Road crossing south-eastwards for nearly a mile along the Southgate Road, there occurs one patch of this material—the ground moraine of an early ice-advance. It includes pieces of chalk and broken flints in a matrix of clay. Evidence of ice transport may be had from pebbles bearing scratches, and the place of origin of these 'erratics’ may be quite distant—purplish Page 3 of 107 quartzites derived from the Bunter (Triassic) rocks of the Midlands, sandstone fragments from the Coal Measures, and rocks carried from the Pennines or even Scandinavia. The most recent deposits have accumulated since the Ice Age, largely as the result of stream action. These deposits represent the old valley floors of streams; they may be found as remnants of terraces on the sides of existing valleys, and are still being formed along river courses. Such deposits are found at a low level in the main valley of the Mimmshall Brook. QUARRIES Most geological strata found in our district have been quarried. Pits have been sunk in the chalk to obtain material for spreading on the heavy clay land to lighten it, a process known as marling, and to provide flints for building. Probably the chalk was also used for making lime. Clays have been extensively used for local brickmaking, though not in recent years, and the very name Potters Bar suggests another obvious use. The thin superficial deposits of sand and gravel have also provided material for building and road-making. SOILS The chalk exposure in the west of the district and the alluvial fill of the Mimms valley carry a medium to coarse well- drained and leached1 ‘brown earth'. The lower valley slopes are rather similar, but the narrow outcrop of the Reading Beds often produces a lighter soil. As all these conditions are favourable for arable farming it is not surprising that the recent Second Land Utilisation Survey found that nearly all the fields surrounding South Mimms were carrying barley or wheat. It was on these soils that the first settlement in the district took place; and a low chalk knoll was chosen as the site for a Norman castle. The characteristic soil of the plateau tops of the district is a leached grey-brown earth suitable for pasture or woodland, the latter sometimes surviving in a semi-natural 1 Leaching is the process by which organic and mineral salts are washed down from upper soil layers by rainwater. state with a typical mixture of deciduous trees such as oak, hornbeam, chestnut, elm and birch. Between these two groups are the ill-drained slopes of London Clay which must originally have been covered with ‘damp oak forest' but which were gradually cleared for grazing and hay. Only on the better-drained parts could crops of corn or ley legumes be grown. The high water table in the London Clay and its poor drainage explain the dominant land use for the production of milk and hay, for both local consumption and the London market. The generally wooded nature of most of our district in early times delayed other settlements for two reasons. The underlying clay which gave rise to the woods was heavy and difficult to plough, and the woodland itself impeded communications. Further, the extensive mediaeval devotion of Enfield Chase to hunting almost sterilised that area for centuries. A few trackways may have been made through the forest, and the names Bentley Heath and Little Heath suggest the existence of natural glades or clearings, but our district has no place names of the distinctive types which indicate very early clearance or settlement.