Figures and maps

(Figure 1) Simplified geology of the Harlech (135) geological sheet showing excursion and foot path routes .

(Figure 2) Generalised stratigraphy and tectonic evolution.

(Figure 3) Stratigraphy of the Cambrian and Precambrian.

(Figure 4) Turbidite facies model. Turbidites form in deep sea fans, many kilometres in diameter, at the mouths of canyons on the edge of the continential shelf. The sedimentological characteristics in a turbidite sequence change away from the source. The sediments near the canyon mouth are generally coarse and show little internal segregation. The sediments of the mid-fan show grading and internal segregation (intervals a to e, see (Figure 9)). Interbedded mudstones and siltstones are progressively more abundant towards the edge of the fan where base-absence sequences predominate (cde) and the sediments pass imperceptibly into the abyssal plain. Near the canyon mouth the inner fan is cut by major distributary channels, whereas the outer fan is mainly an area of accretion cut only by shallow channels not more than 1 to 2 m deep. S. Dzulynski and E. K. Walton (1965) provide a comprehensive description of turbidites in 'Sedimentary features of flysch and greywackes'. Other sources are Ricci-Lucci (1975) and Walker (1965).

(Figure 5) Variations in stratigraphy of the Aran Volcanic Group.

(Figure 6) Stratigraphic range covered by excursions and walks.

(Figure 7) Geology of the Ffridd-brynoch area and excursion route No. 1.

(Figure 8) Geology of the Barmouth area and excursion route No. 2.

(Figure 9) Internal features of turbidites Superficially, turbidite may appear as a monotonous sequence of greenish grey greywackes bed but in detail there is much variation within individual bed . This has been described by a number of author but is now often referred to as the 'Bouma-cycle'. The 'ideal' sequence (i) consists of five intervals labelled a, b c, d , e. Numerous combination of these are possible and some of the most common seen in the Harlech area are shown in (ii to iv).

(Figure 10) Diagrammatic section of turbidite beds above the manganese ore-bed at St John's Church, Barmouth.

(Figure 11) Sketch of turbidite at the base of the Barmouth Formation showing complete 'Bouma cycle' Channel formed base of turbidite bed by current erosion.

(Figure 12) Solid geology and excursion route No. 3 around Clogau and St David's goldmines.

(Figure 13) Cambrian fossils 1 Dictyonema flabelliforme (Eichwald), ×1, from the base of the Dol-cynafon Member of the Cwmhesgen Formation (Tremadoc Series). 2 Parabolinoides bucephalus (Belt), ×1, from the top of the Ffestiniog Flags Formation (Merioneth Series). 3 Olenus micrurus Salter, ×1, from the Maentwrog Formation (Merioneth Series). 4 Homagnostus obesus (Belt), ×4, from the Maentwrog Formation. 5 Tomagnostus fissus (Linnarsson), ×4, from the Clogau Formation (St David's Series). 6 Paradoxides davidis Salter, ×⅓, from the Clogau Formation. 7 Paradoxides hicksii Salter, ×1, from the basal Clogau and top of the Gamlan formations.

(Figure 14) Sketch of contact of dolerite dyke against siltstone in the Gamlan Formation.

(Figure 15) Geology and excursion route No. 4 around Capel Hermon.

(Figure 16) Lingulella daviss (McCoy) from the Ffestiniog Flags Formation. Ventral valve (on left) and dorsal valve (on right), ×3.

(Figure 17) Amphibole-bearing basalt.

(Figure 18) Paths to Llanfachreth over Cerniau and along Bwlch Goriwared.

(Figure 19) Solid geology and excursion route No. 5 at Bryn-llin-Fawr.

(Figure 20) Fossils from the Dolgellau Member 1. Peltura scaraboides (Wahlenberg), ×3, from the P. scarabaeoides Zone. 2. Parabolina spinulosa, (Wahlenberg), ×3 from the P. spinulosa, Zone. 3. Broeggeria salteri (Hull), ventral valve, ×3, from the Acerocare Zone. 4 Niobella homfrayi (Salter), ×2, from the Acerocare Zone. 5 Orusia lenticularis (Wahlenberg), ×3, from the P. spinulosa Zone. 6 Parabolina heres Brögger, ×3, from the Acerocare Zone.

(Figure 21) Geology and excursion route No. 6 in the Allt Lŵyd area.

(Figure 22) Sedimentary structures in the Allt Lŵyd Formation Cross bedding A Thinly interbedded dark siltstone and white sandstone show typical turbulence structures. B A vertical burrow cross-cuts bioturbated siltstone and sandstone.

(Figure 23) Pillow lava.

(Figure 24) Geology and excursion route No. 7 in the Upper Mon Melau valley.

(Figure 25) Geology and excursion route No. 8 around Llartdanwg.

(Figure 26) Geology of the footpath from Barmouth to Hafotty manganese mines (No. 9).

(Figure 27) Panorama Walk, geology and footpath (No. 10).

(Figure 28) Sketch of ice-scour gouge on Panorama Walk.

(Figure 29) Geology of Roman Steps and Rhinog Fawr (walk No. 11) 16 'steps' occur at intervals between localities 2 and 4.

(Figure 30) Geology of Llanbedr Bontddu drovers' route (No. 12).

(Figure 31) Sketch of Pont Scethin.

(Figure 32) Sketch of burial chamber near Tal-y-bont.

(Figure 33) Geology of New Precipice Walk (No. 13).

(Figure 34) Geology of Precipice Walk (No. 14).

(Figure 35) Ffestiniog Flags Formation intruded by a thin sill of microtonalite.

(Figure 36) Ty'n-y-groes to Gwynfynydd (No. 15).

(Figure 37) Pont Dolgefeiliau to Gwynfynydd (No. 16).

(Figure 38) Geology of Torrent Walk (No, 17).

(Figure 39) The Harlech dome circuit.

(Figure 40) Slumped sand stone bed in the Gamlan Formation.

(Figure 41) Fold in the Gamlan Formation.

(Figure 42) Basal Maentwrog Formation at Aber Amffra harbour.

(Figure 44) Simple grading.

(Figure 45) Flame structures.

(Figure 46) Simple grading with large clasts on top.

(Figure 47) Multiple grading.

(Figure 48) Inverted grading at tops of bed (grains have migrated to area of minimum shear).

(Figure 49) Channel formed base of turbidite bed by current erosion.

(Figure 50) High grain concentration at base of graded bed due to shear resistance with substrate.

(Figure 51) Convolute lamination.

(Figure 52) Typical basal section of clinopyroxene crystal.

(Figure 53) Cross bedding.

(Figure 54) Oolitic ironstone. Alternating bands of pyrite, quartz and chlorite form around nucleii of altered feldspar crystals.

(Figure 55) Refracted cleavage in a graded bed.

(Figure 56) Slumped beds.

(Figure 57) Refracted cleavage in a graded bed.

(Figure 58) Cross bedding.

(Figure 59) Simple grading.

(Figure 60) Typical basal section of amphibole crystal.

(Figure 61) Convolute lamination.

(Map 1) Relief map of Harlech Dome area (inside front cover).

(Map 2) Geological map of Harlech Dome area (inside rear cover).

(Front cover)

(Rear cover)

References