Alluvial biota and sections at Broadway West End, Worcestershire in 1988

P. F. Whitehead

The hill stream that passes through Broadway from the hills at Snowshill drains the Jurassic escarpment and sits in a well-defined valley. In the West End area of Broadway at about 90 m O.D. (SP0936) it typically flows in a shallow bed (that is, where it is unmanaged) and has a weakly developed flood plain which reflects its lateral movement in spate. Further downstream it becomes part of the artesian drainage passing under delta-fan gravel aquifers to the north-west of Broadway settlement. Upstream nearer to Snowshill it is floored by thermoclastic scree and there still retains some elements of a more ancient biota. At times in its post-glacial history this stream has been impacted on by the introduction into its catchment of largely soft rock sediments from the escarpment above. The nature and extent of this process correlates with time, climate and human activity. The flow rate and flow characteristics of the stream have been modified over time by these mass movements which grossly modified its sedimentary load and course.

During July 1988 I was able to photograph and draw sections through riparian sediments at Broadway West End whilst construction work was taking place. In seeking to age sediments one requires either artefacts or organisms which are indicative of time or climate. At West End this evidence proved very difficult to find, hence the nature of this discussion. I was able to observe sections up to 2.2 m in depth. The lowest element in all cases was weathered Charmouth Mudstone (Lower Lias Clay) i.e. the solid geology of the immediate area. At one spot evidence was found to suggest that the surface of this may have been exposed previously in a cold climate. A significant surface upbulge and vertical alignment of sediments within that upbulge produced a diapir-like structure. At another spot nearby a seam of well sorted limestone gravel 70 mm thick rested on the Charmouth Mudstone and contained a freshwater mollusc fauna. Above this, beds of dense mottled buff or beige cohesive glutinous clay with limestone fragments reached a total thickness of 1.9 m: this represents an episode during which Jurassic soft rocks were introduced into the catchment in large volume. A hiatus-marking land surface in these clay beds was eventually buried under a further 40 cms of buff clay. At the base of the clay just above the shell-bearing gravels, bones of horse Equus sp. and Red Deer Cervus elaphus L. were found. This clay also contained bones and teeth of domestic oxen Bos sp.. This was 13 m distant from the modern stream. Closer to the stream the ground surface had been built up artificially by tipped sediments in post-medieval time and near the base of this a crude flint scraper was found. Some artefacts were found associated with the early phases of this anthropogenic fill including fragments of hand-blown black glass bottles, Midland Yellow Ware, China Clay pipes and flared glazed bowls dating to the 17th century A.D. This would correlate with early phases of house building in the area.

Molluscs from the basal gravels 2.2.m deep.

Molluscs from these gravels comprise a single shell of the terrestrial gastropod Pupilla muscorum (L.), the hygrophilous Succinea putris (L.), the aquatic gastropod Radix balthica (L.) (= R. peregra), and the bivalves Pisidium amnicum (O. F. Müller) and Pisidium casertanum (Poli). This is the fauna of a well-oxygenated Cotswold hillstream with fringing helophytes and some aquatic vegetation. In other words the aquatic system was relatively stable at that time. Pisidium amnicum is a fastidious species requiring good quality habitat and clean water. The fauna cannot be aged but is of a type whch is apparently no longer extant at the site but which is known to have existed in Cotswold hillstreams more commonly prior to the Bronze Age. This order of chronological magnitude fits well with the evidence for Red Deer just above the gravel. If it is assumed (no more than that) that the gravels are Bronze Age or earlier and the clays no older than Bronze Age, evidence for terrestrial habitat at that time is provided by Pupilla muscorum which is an open grassland indicator species. It is known that as this Jurassic clay was loaded into the catchment by gravity, it was distributed by a stream made much more inefficient, and that P. amnicum persisted for a while in these more turbid conditions before being eliminated.

Chronological hypothesis.

During the present Flandrian Period, hillstreams above Broadway carried clean water and supported molluscs which included the relatively large bivalve Pisidium amnicum. Possible woodland clearance, land use change and increasing climatic oceanicity from the end of the Bronze Age, about 3000 years BP, caused slope instability and failure and the release into the catchment of large volumes of Jurassic soft rock. This seems to have happened relatively suddenly and the valley bottom at West End was finally filled by up to 1.9 m of this sediment. The fact that a land-surface was recognised during this process of deposition does not necessarily have any marked bearing on time, merely on the subsequent movement of the watercourse or change in its nature in response to greater or lesser episodes of slumping and in-wash. The single flint artefact is a very crudely retouched flake without typological significance and which may date to the Iron Age.

Environmental reconstruction.

The aquatic molluscs with P. amnicum are part of the natural post-glacial climax fauna of the relatively stable hill stream. Pupilla muscorum implies open grassland nearby or on the valley side and this is taken to represent a landscape impacted on by humans rather than an early post-glacial one. It is therefore probably one on which trees where scattered or had been cleared, at least on the lower slopes for stock. Domestic cattle remains support this, but Red Deer would imply some wildwood in the area (such as still remain to this day). It is important to stress that there is no evidence of a settled community at this time and that the animal bones are few, not those of extensive occupation. The two associated horse teeth are especially problematical; there is good evidence for the survival of wild horses in this area into the early post-glacial, but what became of them is a highly significant question.