The old-fashioned' County Geology memoir still has life in its bones yet, says author John Catt.
Geoscientist Online Special May 2011
"Oh, no, not
another county geology book!", I can imagine the average GS member saying. True enough,
Hertfordshire Geology and Landscape is just that in part. But the remainder is a detailed attempt to link the geological history of the county to its geomorphology, soils, ecology, agriculture, archaeology, hydrology, water pollution and waste disposal, as a local interpretation of that elusive concept geodiversity.
This broad outline originated with a suggestion made in 1950 by the Council of the Hertfordshire Natural History Society for a book on the county’s geology ‘designed for naturalists who are not geologists’. HNHS was founded in 1875, and from the start had numerous active sections, including a Geology Section with some quite famous geologists as members, such as George Lamplugh, William Whitaker and Sir John Evans. The same structure persists today, though in 2002 the Geology Section separated to form The Hertfordshire Geological Society because of fiscal problems arising from new HMRC regulations relating to charities. Despite the separation, relations between HNHS and HGS are still excellent, and it was appropriate that HNHS should publish the book as one of a recent series on the county, previous examples of which have included a new county Flora, and accounts of local moths, dragonflies, mammals, amphibians, reptiles and birds.
The challenge by HNHS Council was originally taken up by Percy Evans, who had retired to live in Berkhamsted after a career in petroleum exploration. Ably supported by his wife Enid, Percy spent much of the 1950s and 1960s on an intensive programme of geological and other fieldwork across the county in preparation for writing the book. By the time of his unfortunate death in 1973, he had produced partial drafts of about 30 chapters.
Percy’s fieldwork had concentrated strongly on the Thames terraces, older (pre-diversionary) examples of which extend through Hertfordshire to the East Anglian coast. The book could well have been completed before 1973 had not Percy been diverted into correlating this terrace sequence with the foraminiferal and oxygen isotope stages being established by Emiliani and others in Quaternary sediment cores from the Caribbean, Pacific and Atlantic Oceans. The sideline resulted in a long paper published as an appendix to the GS Special Publication The Phanerozoic Timescale. The paper is mainly of historical interest, but demonstrates Percy’s remarkable foresight, because Sir Nicholas Shackleton later developed the study of marine isotope stages so that they now provide the global ‘yardstick’ for Quaternary climatic change, and recent research has shown that the Thames terraces provide the most complete Quaternary sequence known in Britain.
When I inherited Percy’s work in the 1970s, it was clear that his overall broad scheme for the book should be retained, though some chapters could usefully be amalgamated and most needed revision in the light of later research. The glacial rate of subsequent progress leading to eventual publication in late 2010 can be attributed entirely to my failure to effectively pressurize the various local experts I recruited to help with the revision! Nevertheless, much of Percy and Enid’s work remains; in particular, the account of church building stones by Chris Green is drawn largely from their survey of over 200 churches in the St Albans Diocese.
The links with ecology, agriculture and hydrology were through soils, the study of which began in Hertfordshire over two centuries ago, when a local farmer (Arthur Young) published a remarkably accurate map of the county’s soils long before any detailed geological information was available. Later, between 1843 and 1856, Sir John Lawes began long-term field experiments on the growth of various farm crops (wheat, barley, turnips, grass, etc) at Rothamsted, also in Hertfordshire. Most of these ‘classical experiments’ continue today, and since the late 19th Century have been used for government-funded research in numerous agriculture-related sciences, including soil genesis, chemistry, physics and microbiology, water and soil pollution, farmland ecology, crop pests and diseases and biostatistics. Later still, the original headquarters of the Soil Survey of England and Wales (now National Soil Resources Institute) was established at Rothamsted in 1945.
At that time financial stringencies resulted in emphasis on Hertfordshire in early soil mapping exercises, such as the Aylesbury map and memoir. This work in turn influenced development of a national soil classification system and also schemes for either portraying the complex lateral variability of soils on maps at scales of 1:10,000 - 1:250,000 or for defining that variability by geostatistical techniques. The Soils, ecology and agriculture chapter of Hertfordshire Geology and Landscape is based partly on these rich sources of local soils information and partly on the 135 years of detailed observation by generations of HNHS members, as recorded in that society’s publications.
The 94 different soil types (soil series) recognized in the county are distinguished according to profile characteristics, including particle size distribution, mineralogy, colour, stone and carbonate content, pH and aggregate size and shape, in vertical sequences of horizons. These properties, important for agriculture and plant ecology, result from various soil-forming processes (incorporation of organic matter, structural reorganization, weathering, leaching, reduction by waterlogging and illuviation or downward movement of clay in percolating water) acting on originally unaltered sediments (the soil parent materials). Some of the soil parent materials (e.g. Lower Chalk, London Clay and Chalky Boulder Clay) are uniform from the surface down to at least 1.5 m, and typically occur beneath erosion surfaces. But in most of the soil series recognized in the county there is a vertical succession of originally distinct parent materials, such as thin Quaternary deposits overlying bedrock formations.
Mapping of the county’s soil distribution patterns has consequently enhanced our knowledge of later Quaternary history. For example, the topsoil horizons in most soil series are formed in a thin layer of Devensian periglacial windblown silt (loess) or sand (coversand). Also some horizons formed from pre-Devensian sediments are reddened by temperate weathering and enriched in illuvial clay to much greater extents than any soils in Devensian or Holocene materials. These ‘paleo-argillic horizons’ are overlain by the thin Devensian periglacial deposits and are attributed to interglacial soil development, thus indicating land surfaces that survived erosion through at least the Devensian and Holocene. Examples derived from a lithologically variable veneer of Upnor and Reading Formations over Upper Chalk occur extensively on the Chiltern dipslope, where they are shown on BGS maps as Clay-with-flints. Their interglacial origin is confirmed by the presence of Lower Palaeolithic artefacts at numerous sites.
By stretching geology to include these additional topics, our understanding of the county’s landscape development has been deepened considerably. Only time will tell how useful this is to the various HNHS Sections concerned with the county’s biodiversity, but I contend that the approach is essential in exploring its geodiversity.