A summary of the project's phase two fieldwork report, documenting the principal fieldwork undertaken in the Exe and Otter Valleys and at Doniford during phase two of the project, is included below (and a pdf version of the fieldwork report is available here). Full details of the OSL sampling will be included in the dating report to be submitted to English Heritage upon completion of the project (along with the final project report).
Princesshay (Terrace 6)
Excavations by Exeter Archaeology at this site in the centre of Exeter exposed superficial geology (river terrace 6 deposits) and bedrock geology (Whipton Formation and Basalt). According to the site geologist, D. Jordon of Terranova, the bedrock geology slopes gently south west (i.e. slopes towards the present-day course of the Exe). The river terrace deposits exposed at the site occurred at heights of between 40 and 42m OD. According to information from Exeter Archaeology there was a considerable difference between the exposures in the east and the west of the site: in the east the clasts were quite small (c. 10cm), appearing in thin spreads over the site and surviving only in hollows. In the west of the site the clasts were larger (up to c. 20cm) and the deposits exposed were deeper and survived over a wider area. No exposure investigated by the project was greater than 1m in depth, with the river terrace deposits truncated by later human activity.
All of the exposures recorded by the project exhibited significant cryoturbation and the small clast size of c. 10cm is in keeping with the easterly exposures noted by Exeter Archaeology. A sand deposit, related to the terrace, was OSL sampled. The OSL ages of 43,000±5,000 and 44,000±4,000 (OIS-3) are thought to reflect disturbance of the deposit through cryoturbation, rather than the original age of the terrace depositional event(s). Additional data from Exeter Archaeology documented a 1.10m exposure of river gravels (again truncated at the top), with a band of manganese staining at the base. The gravels were sub-horizontally bedded, with variation in the roundedness of the clasts and the matrix, and no evidence for cryoturbation features.
Yellowford Farm (Terrace 6)
Following the excavation of a 20m trench into the terrace 6 deposits at Yellowford Farm, clast analysis (fabric and lithology), organic sampling, and OSL sampling were undertaken. In contrast to Princesshay the river terrace deposits at Yellowford were not truncated and were more extensive and substantial: rather than the very thin drape of gravel anticipated (and thought to be associated with periglacial outwash (Hosfield et al. 2005: 26)), roughly 3.5m of gravel were discovered beneath a topsoil of variable thickness (generally between c. 35-45cm). The gravels may still represent periglacial outwash, and the majority of the deposits were cryoturbated (in-keeping with the Princesshay deposits), with the exception of the deepest gravels in the exposure. An ice-wedge cast was also revealed, indicating cold climatic conditions associated with the formation of the deposit. The depth of the cryoturbation in the exposure at Yellowford suggested that the deposit has probably been repeatedly cryoturbated over several different cold climate episodes, and may imply a significant antiquity for the deposit.
In areas of limited or no cryoturbation, the clast fabric data provided possible indications of palaeoflow directions. Clasts were up to c. 30cm in size, demonstrating at least intermittent high energy conditions in the fluvial system. The lithologies included material derived from Exmoor, probably from the Hangman Formation. Clast analysis also indicated that those clasts from the heavily cryoturbated unit were elongated (slab/rod), which may be related to frost shatter activity.
OSL samples were taken from three areas, although two of these sample locations were interpreted in the field as post-dating the terrace gravels (the third sample was taken from an uncryoturbated unit at the very base of the gravels). The dates have confirmed the post-terrace gravel status of the first two sampled locations (15,000±3,000 and 12,000±2,000). The third date of 78,000±23,000 is more recent than expected, with the large error margins spanning OIS-3 to OIS-5c. Dr Toms however has noted that “Both the accuracy and precision of this sample's age estimate are influenced by two factors. First, the age is based on a limited number of measurements owing to a low mass of datable material. Second, the Shute sandstone may have some influence on the spatial heterogeneity (accounting for age imprecision) and temporal stability of dose rate (accounting for age underestimation through gradual migration towards the sample of relatively high radioactive U and Th bearing material sourced from the Shute sandstone)”. It is therefore apparent that OSL dating is of limited potential in certain areas of the Exe catchment, and that the success of dating high level terrace features in the Exe by OSL is directly influenced by the relatively high radioactivity of the solid geology.
Fortescue Farm (Terrace 4)
c. 3.3m of terrace 4 fluvial deposits were exposed at Fortescue Farm along a c. 20m section. Sedimentary logging, clast fabic and lithology analysis, and OSL sampling (processed ages pending) was undertaken. A series of distinct units were recorded, varying between cryoturbated, chaotically orientated gravels, and sub-horizontally bedded units with no clear evidence of cryoturbation. This suggests that, as at Yellowford, the river terrace deposits have undergone repeated cryoturbation over a succession of different cold climatic episodes. The altitudinal separation of this terrace from terraces 1-3 suggests that it is associated with a major climatic event, which the dates previously known from terrace 3 might suggest to be OIS-4.
Clast lithology suggests (as for terrace 6 at Yellowford) material derived from the Hangman Formation or the Pickwell Down Sandstone on Exmoor (Dr B. Leveridge pers. comm.). There is a clear contrast between clast shape at Yellowford (terrace 6, "sub-rounded" dominant in all units) and Fortescue (terraced 4, “rounded” dominant in all units). There is also a c. 40% reduction in clast size between the terraces (comparing clast sizes across all units), perhaps indicative of changing fluvial processes. Clast fabric data are variable, but the overall pattern is for a dominant palaeoflow direction that differs little from that of the contemporary Exe. (As with the Yellowford data, heavily cryoturbated units are characterised by high dip angles and a greater range of variability in clast orientations.)
Monkey Lane (Terrace 7)
Terrace 7 fluvial deposits were exposed in a 20m trench at Monkey Lane.
The gravels overlay weathered Otter Sandstone, along with a raft of clay derived from the Mercia Mudstone, which occurred at the junction between the gravel and sandstone deposits. The form and size of the mudstone indicated that it was transported as a frozen block, suggestive of cold climate conditions. The gravels had a maximum thickness of c. 1.40m and were clearly truncated. There was no evidence for cryoturbation features as seen at the Exe valley sites, although a clear tree-throw feature was observed. Sedimentary logging, clast fabric and lithology analysis, topographic survey, and OSL sampling was conducted on the main units.
Clast analysis shows no variation in lithologies between the main units: the clasts are completely dominated by locally derived chert and quartzite, with very occasional pieces of flint and sandstone. The fabric data shows some variability, probably related to different palaeochannel flow directions. Three separate sand lenses were sampled for OSL dating. The first of these has been processed and has yielded a date of 140,000±20,000 (OIS-6), but the sample showed some slightly anomalous behaviour and should be considered with caution (processing of the other two samples is pending).
The post-excavation differential GPS survey (post-processed using RINEX data) has revealed a distinct, second break of slope within the terrace (previously mapped as a single unit). Two GPR transects detected both the base of the gravels and variability within the deposits. Of particular interest were a series of anomalies to the east of the site, which may show an additional terrace feature (thus supprting the GPS survey data).
Budleigh Salterton Cricket Field (Terrace 2)
A c. 2.5m exposure of terrace 2 deposits at Budleigh Salterton were logged and sampled for OSL dating (processed ages pending). OSL sampling was from two sand lenses: one within the gravels and one at the base of the gravels at the junction with the underlying Otter Sandstone bedrock. No clear cryoturbation features were observed, although there is root disturbance from the trees above the section. In general the gravel exhibited crude sub-horizontal bedding and clast imbrication, with considerable variability in clast size. No channel features were evident (unsurprising in an exposure of this size), although the sand lenses exhibited clear horizontal bedding. The lithology is overwhelmingly dominated by clasts from the Budleigh Salterton Pebble Beds, with occasional flint and chert clasts (as for Terrace 7 at Monkey Lane: see above).
Additional Sites in the south-west region
Doniford (Paleoriver Washford), Somerset
River terrace deposits (related to the palaeoriver Washford) exposed in the Doniford cliffs have yielded handaxes and mammoth remains in the past. In light of the extensive recent erosion of the cliff sedimentary logging, organics sampling, and OSL sampling was undertaken.
The deposits at this site are complex: some units are very disturbed by cryoturbation and contain clasts that have been split in situ by freeze/thaw processes. Other units are clearly bedded and have not been affected by cryoturbation. OSL samples were taken from four locations, yielding ages of 51,000±4,000, 49,000±5,000, and 45,000±5,000 on the basal gravels, and 25,000±3,000 higher up the sequence. These ages could suggest differently aged units with the cliffs, but our currently preferred interpretation is that the river deposits were reworked several times (the OSL ages therefore date the latest reworking event rather than the depositional activity associated with the initial inclusion of the artefacts). It is probable that the Doniford artefacts were originally derived from the Washford Valley (and may be contemporary with terrace 2 of the River Tone, where similar artefacts have been found).
In all cases pollen was poorly preserved within the organic samples (no other organic materials were recovered). Where pollen grains could be identified, cold species were represented, but in no cases were the quantities statistically significant.
The fieldwork has highlighted the evidence for significant Pleistocene inter-valley variation: while the Rivers Exe and Otter are clearly staircase strath terrace systems, the Axe is a fill terrace system. The gravels in the Axe are made up of different units (recorded in the sedimentological logging), previously mapped as single terraces. The geomorphological mapping and modelling have also highlighted altitudinal differences (i.e. breaks of slope) within 'single' terraces, further suggesting multiple units within compound terraces.
The difference between the fill terraces of the Axe and the staircase terraces of the Exe and Otter may well be related to an abundant local supply of clasts from the weathered Cretaceous and Tertiary material of the Blackdowns, and a lack of main channel discharge capable of removing this material and incising (until the downcutting that occurred when the last glacial sea level was reached).
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