The Flag Fen Auger Survey

Abstract

The Flag Fen auger survey was commissioned by the Flag Fen Futures Group and funded by Peterborough Limited and the McDonald Institute’s Fenlands Futures Archaeology and Heritage Research Initiative (FFAHRI). The objective of the auger survey was the creation of a definitive model of the Flag Fen Basin in the vicinity of the Flag Fen post-alignment and platform corridor; accurately mapping the pre-Holocene land surface and characterising the surviving Holocene sediment sequence. The auger survey involved a 1100m x 500m grid of 235 individual auger points spaced at 50m intervals. The survey grid encompassed the 1.1km long path of the Flag Fen post-alignment/platform, starting at Fengate (west) and ending at Northey Island (east). In the event, the survey successfully achieved a total of 200 points (issues of access, concrete culverts and high-pressure gas mains precluded the remaining 35 points). The largely soft, yet cohesive deposits that characterised this part of the Flag Fen Basin proved to be particularly amenable to the hand auger survey methodology. Beyond a dry, friable plough soil and compacted alluvial cover, the deposit sequence comprised mostly malleable peats, silty peats, organic silts and sandy clays. A buried soil horizon was recorded in the majority of the auger holes. The base deposit involved clayey gravels (March Gravels) which proved to be generally impenetrable with the hand auger. The deepest auger hole was 3.8m, the shallowest 0.2m (average depth: 1.46m). The lowest recorded point in the basin equalled -1.24m OD, whilst the highest equalled 2.87m OD, a difference of 4.11m. The resulting model established an asymmetrical, bowl-shaped profile with a clear dip towards its eastern end. In plan, the model showed the basin to have a relatively gentle gradient across its western half (the highest contour equalling 0.80m OD, the lowest 0.20m OD), beside a 250-300m wide ‘valley’ that bottomed out at about -1.00m OD across most of its eastern half. The relative steepness of the adjacent island edge of Northey (0.00m OD to 2.00m OD in approximately 100m) further exaggerated the basin’s asymmetric profile. By way of contrast, the landfall towards Fengate was far less dramatic, although it did include another slight depression near the basin’s edge. A narrow promontory, situated very close to the route of the western stretch of the post-alignment was another distinct topographical feature. Unquestionably, the two most significant outcomes of the new model are: 1) confidence in its authenticity in comparison to all previous models of the monument’s setting, and, 2) the very evident interrelationship between the projected location of the Flag Fen platform and the location of the deeper and wetter valley zone situated along the eastern margins of the basin