The Vale of Ffestiniog 16

Geology Field Studies from Lleyn to Plynlimon The Vale of Ffestiniog 16 Lower Rhyolitic Tuff Dol-cyn-afon sandstone, siltstone, mudstone Cwm Eigiau mudstone, siltstone Dolgellau and Ffestiniog formations Maentwrog formation Nant Ffrancon siltstone Hafotty, Barmouth and Gamlan formations Microgranite Figure 458: Rhinog formation Intrusive rhyolite Field excursions. In this chapter we examine the northern margin of perhaps as fault movement was transferred to the the Harlech Dome, between the outcrops of the fractures which run through central Snowdonia Cambrian Welsh basin sedimentary sequence to and Cwm Pennant. The sea bed in the vale of the south and the Ordovician Snowdon volcanics Ffestiniog area was covered by uniform deposits of to the north. volcanic ashes, along with sand and mud sediments. The vale of Ffestiniog lies along the line of a presumed deep fracture in the crust. There is In the later Ordovician, volcanic activity reached a sedimentary evidence that this fracture formed a climax in Snowdonia. Magma rising from a deep break of slope in early Ordovician times, with delta reservoir could follow the fractures deep beneath deposits produced by rivers flowing southwards the vale of Ffestiniog to reach the top of the into the marine Welsh basin (fig.459). Cambrian sedimentary succession and emplace the large microgranite intrusion of Tanygrisiau. This It is likely that the vale of Ffestiniog fracture zone intrusion contrasts with the majority of Ordovician became inactive by the middle Ordovician, felsic intrusions in central Snowdonia in having a 264 Chapter 16 The Vale of Ffestiniog coarser grain size, allowing individual crystals of a major volcanic centre developed nearby at feldspar and mafic minerals to be seen with the Manod near Blaenau Ffestiniog, and ashes were naked eye, in contrast to cryptocrystalline erupted in the Moelwyn mountains above intrusions elsewhere. This is evidence that the Tanygrisiau. These events will be discussed in Tanygrisiau intrusion solidified beneath the ground chapter 17. surface without losing steam through a volcanic The vale of Ffestiniog is mainly underlain by vent, promoting the growth of larger crystals in Cambrian and lower Ordovician sandstones, the hydrous melt. siltstones and mudstones. This succession is A feature of the Tanygrisiau intrusion is a very intruded by a series of cone sheets originating large metamorphic aureole surrounding the from the Snowdon caldera. Lead mineralisation is microgranite, where mudstones have been present, associated with mountain building during recrystallized to hornfels by heat and the action of the Devonian period, and has been worked in steam escaping from the solidifying intrusion into mines around the village of Rhyd, and in the the country rock nearby. Moelwyn mountains. The vale of Ffestiniog did not completely escape the effects of late Ordovician volcanicity, as Lower Ordovician Moelwyn area: coastal plain and Allt Lwyd formation shallow marine shelf delta deposits Figure 459: subsidence of the Evolution of the Tanygrisiau trough Vale of Ffestiniog. deep crustal fracture zone zone of contact metamorphism, Upper Ordovician converting mudstones to hornfels Microgranite intrusion Ordovician basin sediments Tanygrisiau microgranite deep crustal fracture zone, with faulting inactive after the early Ordovician crust northwards beneath Avalonia. The overall By Silurian times, the microcontinent of Avalonia effect of these plate tectonic processes was to was converging with the major continent of cause compression and crustal shortening in the Laurentia, assisted by subduction in the Rheic Welsh Basin. These movements affected the ocean to the south which was carrying oceanic thinner and softer bedded strata near the surface, 265 Geology Field Studies from Lleyn to Plynlimon north south folding of bedded strata north-south crustal shortening through transverse faulting vertical block faulting in the Figure 460: Precambrian basement Crustal compression and folding in the Welsh Basin. but were largely controlled by strong and massive movements. Compression of strata in the Vale of blocks of the Precambrian basement underlying Ffestiniog reached such an intensity that Wales. Both horizontal and vertical fault movements could no longer be accommodated by movements could occur between basement blocks folding, and multiple low angle thrust faults (fig.460). developed. An example near the village of Rhyd is shown in fig.461. The Vale of Ffestiniog fracture zone appears to have played an important role as a boundary Field excursions in this section include: between the upfaulted crust of the Harlech Dome Porthmadog and the coast of the Traeth Mawr and to the south, and downfaulted crust beneath Traeth Bach estuaries; the mountains above Snowdonia to the north. In addition, crustal Maentwrog; and the outcrop of the microgranite shortening occurred through transverse fault at Tanygrisiau. NORTH-SOUTH CRUSTAL SHORTENING THRUST HARLECH DOME UPLIFT SNOWDONIA SUBSIDENCE Moelwyn Dol-cyn-afon slates and Garth Grit volcanics Garth Flags Figure 461: Thrust faulting at Rhyd, leading to repetition of Garth Grit the outcrop of Garth Grit. Garth Grit THRUST Garth Moelwyn Moelwyn Garth Grit Garth Flags Garth Grit Flags slates volcanics 266 Chapter 16 The Vale of Ffestiniog Penrhyndeudraeth 3 miles: approximately 1½ hours © Crown copyright 2019 OS 100061048 Moelwyn tuffite Microgabbro Nant Ffrancon siltstone Intrusive rhyolite Garth Grit sandstone, conglomerate Dol-cyn-afon mudstone, siltstone Figure 462: Field excursion. Dol-cyn-afon sandstone Dolgellau mudstone, siltstone 1: From the car park, take a minor road In this excursion we visit an area of former westwards up a hill. At the junction, turn left to coastline of the Traeth Mawr estuary where a Pen-y-Bryn, then north along a minor road which series of rocky ridges once extended into the sea passes under the Ffestiniog railway. as headlands and islands. These ridges are now separated by flat alluvial fields. We will see 2: Take the footpath alongside the railway, then evidence of the enormous thrust faulting which through the woods to Ty-fry. At the footpath occurred on a mountain building scale along the junction, take the left branch over the hill to Ty Vale of Ffestiniog during the late Silurian and early Obry past outcrops of Garth Grit. This coarse grit Devonian periods. occurs near the base of the Allt Llwyd formation. It was one of the first deposits laid down in the Start: A car park in available in Welsh Basin as subsidence began again at the start Penrhyndeudraeth [SH611391]. of the Ordovician period, following uplift, erosion, and early volcanic activity at the Rhobell Fawr centre. 267 Geology Field Studies from Lleyn to Plynlimon The Garth Grit at this point was deposited in a around the ridge of Ynys Berfed. Observe the series of delta fans, extending from a shoreline outcrops in the roadside cuttings. along the northern margins of the Vale of A panorama opens up across the valley (fig.463) at Ffestiniog, southwards into deeper waters in the Ty Obry. The upper crags along the ridge of Ynys Harlech Dome. We see thickly bedded materials Berfed are formed by a thick sill of rhyolite. The with pebbles present in places. Rates of sill is intruded into ashes of the Moelwyn Volcanic deposition were high, as large quantities of coarse formation which dip to the north and overlie older sediment were carried down large rivers during Ordovician slates. As we shall see, the sequence storm events over the mountains to the north. has been strongly affected by earth movements. 3: At Ty Obry, join the minor road which skirts Figure 463: Ynys Berfed ridge from Ty Obry rhyolite sill ash crushed slates rhyolite sill The first outcrops seen along the roadside are angle thrust planes cut across the cleavage. Blocks bedded fine rhyolitic ashes of the Moelwyn of rock from disrupted volcanic ash bands are formation. These ashes were erupted in late mixed with the slates. Ordovician Caradoc times, but from a localised volcano not forming part of the Snowdonia Detailed mapping has shown that Ynys Berfed is caldera. A likely vent for this volcano, infilled with cut by several thrust faults, so that many of the agglomerate, has been identified on the hillside to geological boundaries we see are faulted. A the north-east of the Ffestiniog Railway station at boundary zone between slates and the sill Tan y Bwlch. intrusion contains a breccia composed of keratophyre fragments in a slate matrix, produced The Moelwyn ashes are underlain by slates which as the rock masses moved over one another in the are heavily deformed. In places the slate breaks thrust zone. into pencil-like fragments, and many small low- THRUST sill intrusion THRUST Figure 464: Geological THRUST structure of the western end of the Ynys Berfed ridge. crushed slate breccia of slate and keratophyre 268 Chapter 16 The Vale of Ffestiniog Figure 465: (top left) Rhyolitic ashes of the Moelwyn Volcanic formation (top right) Crushed slates of the Nant Ffrancon formation. (bottom right) Fragment of a volcanic ash band within the crushed slate. The upper crags of the Ynys Berfed ridge are albite feldspar. The rock type is classified as formed by a felsic sill. This is a very fine grained keratophyre due to high sodium content of its rhyolitic rock containing phenocrysts of white feldspar. Figure 466: Keratophyre sill. 269 Geology Field Studies from Lleyn to Plynlimon Figure 467: (left) The Hir Ynys ridge, formed by a microgabbro sill. (right) Detail of the microgabbro. At the top of the ridge, observe the crags of Follow the road through the hill gap at Bwlch- 4: microgabbro to the left of the path. The ridge of Y glas and continue north to Hir Ynys. Garth is again formed by a cone sheet intrusion, similar in composition to sill at Hir Ynys. A short 5: The Hir Ynys ridge is again formed by a distance through the woods to your right, beyond northwards-dipping sill intrusion, this time of mafic the 'DANGER' signs, is the working face of the large composition.

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