Date and Time
Will be provided on registration
Field Trip Description
Saturday 2nd July –Eastern Forest
- Flaxley and Mugglewort Wood (Silurian, Wenlock and Ludlow, Malvern Fault, Severn Vale)
- Upper Soudley (Lower ORS, Upper ORS to Lower Carboniferous, intra-Devonian unconformity)
- Lydney Shore (Lower ORS, structures)
Sunday 11th September –Central and Western Forest
- New Fancy (Upper Coal Measures – Supra Pennant, Forest Geo-Map, and views)
- Mallards Pike to Howbeech Slade (Lower Carboniferous, Upper Coal Measures – Trenchard and Pennant, intra-Carboniferous unconformity)
- St Briavels (Upper ORS to Lower Carboniferous, Wye valley geomorphology, industrial geology)
GEOLOGY OF THE FOREST OF DEAN
The Forest of Dean forms a relatively geographically isolated area, partly due to its location between the Rivers Severn and Wye, and partly due to its geological structure, formed by modern erosion into a nearly continuous, difficult- to- penetrate barrier of encircling ridges around its margins. The sedimentary rocks of Silurian, Devonian and Carboniferous age were folded into a complex, basin-shaped syncline, with north-south axis, during the Variscan orogeny that saw the collision of Laurussia and Gondwana to form Pangea, during and at the end of the Carboniferous period, about 300Ma ago. The folding took place largely due to movement on the adjacent, massive, Malvern Fault system to the east – the deformation is much more severe on the eastern than the western limb, producing a highly asymmetric structure. The movement took place in the form of thrusting from the east in two main phases; the more important in Late Dinantian to Westphalian times, equivalent to the Sudetic Phase of the main part of the Variscan belt; the later at the end of the Carboniferous, post Coal Measures, equivalent to the Asturian Phase.
Silurian rocks, of typical tropical shelf facies, if not thickness, outcrop in the anticlinal inlier of May Hill to the NE of the main syncline (Day 1, Loc. 1). These are followed by Old Red Sandstone terrestrial facies mudstones with sandstones and calcretes of Pridoli age,, forming a low-lying vale outside the main rim to the coalfield (Day 1 Loc 3). Progressively coarser (Devonian) ORS sediments form three prominent ridges around the Forest edge (the Chapel Point Calcrete, the lower part of the Brownstones [Senni Formation of Davies 2017], and, on top of the parallel unconformity marking the Acadian Orogeny, the Huntsham Hill Conglomerate [Quartz Conglomerate of old])(Day 1 Loc 2).
Conformable on the Old Red Sandstone are the lower Carboniferous sediments, showing evidence of near-shore deposition, with much sandstone and dolomitisation of the limestones. These resistant rocks add breadth to the ridge formed by the Upper Old Red Sandstone. (Day 1 Loc 2, Day 2 Loc 3). The rest of the Dinantian, all of the Namurian and all of the Lower and Middle Coal Measures are missing, either never deposited or removed by erosion following Variscan (Sudetic) uplift and folding. The Upper Coal Measures were then, with great unconformity, laid on top (Day 2 Loc 2), and gently folded into a synclinal basin by the late Carboniferous Asturian phase of the Variscan orogeny.
Only two of the many coal seams were of any thickness, and were the main focus of underground mining (the Coleford High Delf and the Yorkley), though many of the thinner seams have been worked on a small scale in the past and briefly in the 1960s to 80s by open cast quarrying. The mining and processing of iron ore (goethite) has also been an important industry in the Forest, the ore probably being derived from Permo-Triassic weathering of pyrite and siderite in the Coal Measures, and its emplacement into the underlying rocks, especially the Dinantian limestones, by descending acidic meteoric water. (Day 1 Loc 2, Day 2 Loc 3). Many of the local rock types have been used for building, including cement from the Lower Limestone Shales, and brickmaking from the Coal Measures shales, but one rock type is of exceptional value as a building stone – the olive green Pennant Sandstone, which has been quarried on a vast scale. Today, there is little extractive activity apart from small scale building stone quarrying and processing, and extraction for aggregate of the dolomitised Dinantian limestones (2 active quarries).
The scenery of the Forest of Dean is mainly closely related to the underlying geology (Day 1 Loc 1). Following recent ( 400,000-500,000years ago?) uplift of a (Oligocene-Pliocene?) near sea level erosion surface cut across all the rock types, the weakest rocks (the Permo-Triassic and Liassic muddy sediments of the Severn Vale, the poorly cemented sandstones and mudstones of the Old Red Sandstone [Raglan Mudstones and Upper Brownstones] of the outer rings, and the mudstones of the Supra-Pennant coal measures in the centre of the basin) have been eroded down into low ground. The more resistant rock types (Silurian limestones and sandstones of the May Hill inlier; calcretes, conglomerates and well-cemented sandstones of the ORS plus the lower Carboniferous limestones forming the outer ridges; and the Pennant Sandstone forming a massive inner ring) have been left standing up as concentric ridges and escarpments, scalloped into promontories and embayments to north and south by subdsidiary Sudetic folds.
In contrast, much of the drainage seems entirely unrelated to the underlying geology and structure. In particular, the famous incised (actually “entrenched”) meanders of the river Wye, presumably were initiated into alluvial deposits on the near sea level erosion surface, their size indicating huge (meltwater?) discharges. Downcutting began as uplift started, the river encountering different rock types, whose resistance to slope processes determined the valley profile (gorge to open valley) as deepening progressed. The valley equivalents of ox-bow lakes were created in three places during this process as meanders were cut off (Coughton, Newland and St Briavels [Day 2 loc 3]). Smaller streams within the Forest also largely ignore the underlying structure, such as the east-draining Westbury Brook and Longhope Brook (Day 1 Loc 1), which flow across the steep eastern limb of the syncline, and show similar, if smaller, incised, meandering and misfit valleys.
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