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Thursday 4th July
We visited Heybrook Bay in
order to study the geological
features and understand the processes
that formed them.
The study area was Renney Rocks in Heybrook Bay. In terms of geology, the beach consists of mudstones and sandstones, deposited by a river during the Devonian period. There has been plenty of geological activity, evidenced by various faults and folds observed on the day.
Our first task was to identify and map the dip and strike of some of the geological folds present. The beds were found to have been folded to form a dip of between 34° and 42°, with a strike angle ranging between 208° and 226° (SW). Stress fractures were identified near the axial trace of the antiforms, which had either been filled in with quartz. A dextral fault (aka right lateral) was also observed. The fault displacement was approximately 6m horizontally with no vertical movement.
After this, a section of the upper bed was observed in detail. The bottom layer was composed of a conglomerate (1). This was clast supported by a mix of angular and some rounded pebbles. The next bed was a thin strata of brown clay, approximately 30cm thick (2). Above this lay a 1.5m thick bed of silt with several angular boulder sized clasts (3). This was poorly sorted.
The upper section of the bed was formed of four sections. The first of these represents a discontinuity, formed of a 4cm layer of clast supported breccia (4). The clasts were highly angular, with an approximate size of 2.5cm. Above this, the loose brown clay layers were once again found. A clast supported breccia was found (5), which exhibited very poor sorting, with gravel sized clasts. This was found with a light brown clay matrix. The final bed found beneath the topsoil was a 1.3m thick bed of clast supported breccia (6). This showed moderate sorting in gravel sized clasts. The matrix was a darker brown suggesting a different origin from the previous beds.
Above that were some alluvial layers, as seen by the presence of conglomerate. Other layers appeared to have been formed as a result of glacial activity, with glacial runoff being a major layer of these beds, as seen by poor sorting and angular boulders. This layer could have also formed in a mudflow.
