The Pleistocene Stratigraphy and Palaeoenvironments of the Cambridge District
Between 1995 and 2002, Steve Boreham studied part-time for an Open University PhD, a summary of which is below, supervised by Dr Charles Turner (OU) and Dr Colin Forbes (external).
- Aims of the project
- Research activities
- Literature searches and visits
- Fieldwork and site visits
- Laboratory analyses
- Computer analyses
- Landscape change
This study attempts to record and interpret Pleistocene geological data from the Cambridge District in a comprehensive format. The author has established a central study area comprising twenty 5km squares, embracing part of the lower Ouse valley and much of the Cam valley. This area has yielded some 5000 individual existing borehole and trial pit records. In addition, about a hundred boreholes and trial pits have carried out during this project. These have yielded detailed stratigraphic data, and more than three hundred sediment samples to which a variety of laboratory analyses have been applied. The author has used a novel rigorous lithostratigraphic approach as the primary line of evidence for correlation in this study, in place of the traditional morphostratigraphic paradigm. Existing stratotype localities in the study area have been reviewed and modified where appropriate, and several new stratotypes for lithostratigraphic members have been proposed.
Many classic Cambridge sites including Traveller's Rest Pit, Histon Road, Barnwell Abbey, Barnwell Station, Grantchester and Barrington have been re-evaluated and new exposures investigated wherever possible, allowing the construction of databases of stratigraphic, physical and palaeontological information from a myriad of sources. Geochronological techniques including OSL, radiocarbon dating and aminostratigraphy have also been used for the correlation of deposits, together with biostratigraphic evidence. The presence of deposits from more than one temperate stage has been confirmed or strongly inferred for many of the major sites investigated. The relative antiquity of deposits at the Traveller's Rest Pit has been confirmed, and the dating of the most recent phase of temperate deposition at Barrington and Histon Road established as Ipswichian (MIS 5e). In contrast, the majority of deposits at sites such as Grantchester and Swan's Pit are strongly correlated with MIS 7. This study has enabled confident correlation of Anglian glacial, and post-Anglian fluvial deposits of the Great Ouse and Cam systems within the Cambridge District. It has also allowed the correlation of these deposits with others elsewhere in southern England. However, the reworking of faunal and floral remains between interglacials has been highlighted, and has major implications for biostratigraphy.
This study has unravelled a complex Pleistocene geological history including the advance of the Anglian ice sheets, the genesis of the Ouse and Cam river systems, major landscape change and the development of the drainage patterns observed today. The Ouse and Cam river systems initially developed along lines of Anglian sub-glacial drainage. In cold stages these rivers occupied high-energy braidplains characterised by episodic erosion, and the aggradation of gravels and sands. In temperate stages, they occupied relatively stable low-energy channels, resulting in the accretion of fines and organics. The study has also allowed the interpretation and understanding of many different Pleistocene palaeoenvironments within the Cambridge District including, subglacial, proglacial, tundra, scrub-tundra, steppic grassland, boreal forest and temperate woodland.
Aims of the project
The chief aim of the project was to describe, record and interpret Pleistocene stratigraphic, geological and palaeontological evidence from the Cambridge District to enable the reconstruction of past vegetation, climates, environments and landscapes. It is hoped that this work will lead to the creation of detailed geological maps of the Cambridge District, and a greater understanding of the chrono-stratigraphic position of the various Pleistocene deposits in the area, allowing correlation with British and European sequences.
The research activities which were undertaken during this project involved the collection of geological, stratigraphic and palaeontological data from a variety of sources. This may be summarised as data acquisition from three main routes; literature searches and visits, fieldwork, and laboratory analyses. As a consequence of this work, databases for each main data type were created, and considerable investigation into appropriate software capable of two- and three-dimensional data representation was made. In addition, broader issues concerning the development of the landscape in the Cambridge District over the Pleistocene in relation to different bedrock types and overlying deposits were addressed.
Literature searches and visits
This type of data acquisition may be divided into visits to libraries and library database searches for relevant scientific papers and reports, postal campaigns, visits to obtain data from surveyors, civil engineers and other organisations, and visits to museums to collect data from records and specimen collections.
Libraries which were used to obtain papers and reports include those of the Department of Geography, Department of Plant Sciences, Department of Archaeology, Scientific Periodicals Library and University Library (University of Cambridge), Cambridge City Library and British Geological Survey Library. There are also a number of on-line services which were useful for searching for references. A wide range of data was obtained in this way, including stratigraphic, geological and palaeontological information. A collection of relevant papers, reports and publications has been assembled.
There were several postal campaigns aimed at obtaining stratigraphic and geological data from surveyors, civil engineers and various other organisations and individuals in the form of borehole and trial pit records from technical reports. At first, a range of sources were identified from business databases and professional contacts. Each source was first telephoned to ascertain the name of an appropriate person to whom a standardised letter was sent. This often resulted in an invitation to visit to collect information from the organisation, or information being sent by fax or post. Every contact was asked to suggest other people who might also be able to assist, so that a list of more than a hundred respondents was built up, giving rise to the acquisition of many hundreds of separate borehole and trial pit records.
The Quaternary Museum, now housed in Department of Geography, and the Sedgwick Museum, Department of Earth Sciences (University of Cambridge), were visited with a view to the collection of particularly palaeontological data from various records and specimen collections. These included large and small mammal faunas, herpetological and fish faunas, molluscan faunas and plant macrofossils. Sadly, there was no functional computerised index system at either of these museums. However, a laptop computer has proved especially useful for recording the collections which are housed in a large number of storage drawers and cupboards.
Fieldwork and site visits
Data acquisition from fieldwork entailed visits to building sites, developments, sewer repairs, gravel pits and temporary excavations to obtain engineering reports containing borehole or trial pit records. Fieldwork also involved recording the stratigraphy and taking samples for laboratory analysis from exposed geological sections at these sites, and carrying out investigative visits to put down boreholes or trial pits using a variety of drilling and excavating equipment. It was usually necessary to obtain written permission to visit and work on field sites. Building developments and repairs to the infrastructure of Cambridge have constantly provided opportunities to collect geological information. Almost every development, through necessity, has boreholes and trial pits associated with it. These formed a prolific source of stratigraphic data throughout the study period. There were more than a hundred visits of this type, again giving rise to the acquisition of many hundreds of borehole and trial pit records.
In many instances these records were augmented and verified by direct observation of geological sections at the field sites. On frequent occasions, it was also possible to make measurements and take samples from the exposed deposits for subsequent laboratory analyses. More than two hundred sediment samples were collected in this way. At some development sites it was possible for me to carry out boreholes or trial pits. However, investigative site visits were concentrated in areas where there are few other records, or where there are conflicting or ambiguous stratigraphic details. About fifty boreholes and trial pits were carried out during this project, which have yielded stratigraphic data, and many samples for laboratory analyses.
The stratigraphic position, physical size and nature of the samples collected from field sites governed which laboratory analyses were appropriate. In many cases it was only been possible to collect, at best, several bulk samples from exposed sections in excavations due to the confined and dangerous working conditions In these situations, representative samples of the main stratigraphic units were taken. Occasionally, there were opportunities for closely spaced sampling from exposed sections. In these cases, discrete and bulk samples were taken together to allow a greater variety of analyses to be carried out. Finally, the samples originating from boreholes have tended to be small and closely spaced, and there was often little possibility for procuring bulk samples in this situation.
Bulk samples of gravel and sand present perhaps the least opportunity for laboratory analyses, although sieving and clast lithological analysis (CLA) can be applied to them. More than seventy samples of gravel-rich sediments were analysed in this way, and CLA has proved to be a powerful tool in distinguishing between otherwise superficially similar coarse-grained sediments. Some bulk samples, especially those of fine-grained deposits, yielded mammal remains, molluscs, beetle fragments and plant macro-fossils when they were sieved. Sub-samples of bulk samples were been subjected to the analyses described below, although the stratigraphic control was often poor. At a number of key localities, for example at the Histon Road Allotments site, boreholes provided relatively long sequences of between twenty and forty contiguous samples. These samples were subjected to a rigorous suite of physical analyses including loss-on-ignition (LOI), magnetic susceptibility (MS), laser particle size distribution (PSD), and pollen analysis (PA). In these situations, it was possible to discover a great deal about the depositional history of the sediments. Shorter sample sequences were also prepared, so that about a hundred samples were analysed in this way. These types of analysis were extremely time-consuming, so that only a limited number of samples could be dealt with at any one time.
Three main databases were created as a result of the data acquisition efforts detailed above. The stratigraphic database contains all of the borehole, trial pit and other stratigraphic records gathered from a myriad of sources throughout the project. There are at least 1500 individual records from the Cambridge area. It proved necessary to divide the district into tetrads (4km2 areas) in order to handle this volume of data. For some of these tetrads, various attempts were made at mapping the Pleistocene deposits using specialised software. The geological database is essentially a collection of smaller datasets including analytical data from CLA, LOI, MS and PSD. These data were used to correlate deposits from different sites, and to interpret environments of deposition. The palaeontological database includes information from published records, museum collections, laboratory analyses of plant and animal remains, and the results of pollen analysis. The faunal and floral inventory created for each site gives a profile of the vegetation, environment, climate and possibly age of the deposits.
The collection, cataloguing and processing of large amounts of data was made easier by recent advances in personal computer technology. The three databases outlined above required a relatively sophisticated combination and interpretation to provide a balanced and defensible set of conclusions. Considerable efforts were made to find and trial appropriate software capable of data analyses, and two- and three-dimensional stratigraphic data representation using the databases, with a view to including these in my thesis. The essence of my findings in this area were published in Quaternary Newsletter (February 1998 No.84) in an article entitled 'Three-Dimensional Stratigraphic Data Representation Using a Personal Computer'.
Throughout the study period, attention has been given to the problem of how the landscape of the Cambridge District has evolved over the Pleistocene Epoch. The study has concentrated on the apparently large amount of bedrock wastage due to periglacial processes, and in particular thermokarst. These findings were presented at a Periglacial Workshop in Cardiff in December 1997, and have been published in an article entitled 'Thermokarst landforms in the Cambridge area' in Nature in Cambridgeshire (1996 No.38). Work is being done on the possibilities of combining models of bedrock wastage with observed stratigraphic data using software capable of three-dimensional manipulation to create reconstructions of changing Pleistocene land surfaces.