Editing Moel Hebog (section)

==Geology==

Moel Hebog stands at 2,566 feet (782 metres) above sea level in North Wales, forming a major peak in the region between succession of [[Cambrian]] and [[Ordovician]] rocks that have been extensively studied to understand the geological evolution of North Wales.<ref name="Shackleton 1961"/> On Moel yr Ogof can be found Owain Glyndŵr's Cave.<ref name="Marsh 1984"/><ref name="Marsh 1985"/><ref name="Nuttall 1999"/>

===Stratigraphy===

The [[stratigraphy|stratigraphical]] sequence begins with the [[Inliers and outliers (geology)|inliers]] throughout the area. These beds consist of greenish‑grey to bluish‑grey slates with distinctive layers of [[siltstone]] locally known as "ringers". In the northern exposures, these beds also contain distinct [[quartzite]] layers, some of which are pebbly near their tops. The Ffestiniog Beds are topped by the Lingulella Band, a horizon about {{cvt|55|ft}} thick containing abundant specimens of the [[brachiopod]] species ''[[Lingulella davisi]]''. Above this, in some locations, lie the Dolgelly Beds, which contain a diverse [[fauna]] including [[trilobite]]s such as ''[[Briscoia celtica]]'', ''[[Conokephalina abdita]]'' and ''[[Parabolina]]'' species, along with brachiopods like ''[[Orusia lenticularis]]''. These fossils indicate the lower portion of the Dolgelly Beds, specifically the zone of ''[[Parabolina spinulosa]]''.<ref name="Shackleton 1961"/>

[[Tremadocian]] rocks extend into the area along the Ynyscynhaiarn [[anticline]], with the most fossiliferous exposures showing delicately striped leaden‑grey blocky [[mudstone]]s containing trilobites such as ''[[Asaphellus homfrayi]]'' and ''[[Agnostus calvus]]'' and various brachiopods. These mudstones belong to the Portmadoc Flags Formation, a locally recognised Tremadocian unit of thinly bedded, fossil‑rich mudstones and siltstones deposited in an offshore marine environment. The junction between Cambrian and Ordovician rocks remains uncertain, as many contacts appear to be [[fault (geology)|faulted]] or slipped, though the overall pattern shows Ordovician rocks overstepping Cambrian towards the north‑west, a trend seen throughout [[Caernarfonshire]].<ref name="Shackleton 1961"/>

The Ordovician sequence begins with the Pennant Slates and Quartzites, followed by the Maesgwm Slates of cleavage characterised by closely spaced, discontinuous partings that produce thin, flake‑like layers—grading upward into softer slates showing streaky alternations of dark grey and greenish‑grey material. These beds have yielded ''[[Didymograptus murchisoni]]'' and ''[[Didymograptus geminus|D.&nbsp;geminus]]''.<ref name="Shackleton 1961"/>

===Volcanic and intrusive rocks===

A significant feature of Moel Hebog is the [[Snowdon Volcanic Group]], which forms much of the mountain's upper reaches. The volcanic sequence closely resembles that on [[rhyolite|rhyolitic]] series separated by a basic series. It begins with the distinctive [[feldspar|feldspathic]] streaks showing characteristic {{cvt|5|ft}} across, basic tuffs and the [[Moel yr Ogof]] Basalts—a sequence of at least five [[basalt]]ic lava flows well exposed in cliffs on the south side of Moel yr Ogof. Chemical analyses show these to be distinctly alkaline [[syncline|synclinal]] outlier south of the summit of Moel yr Ogof.<ref name="Shackleton 1961"/>

The area contains numerous [[igneous intrusion]]s of two main age groups: those related to the [[Caradocian]] Snowdonian [[volcanism]] and those [[pluton emplacement|emplaced]] during the [[Caledonian orogeny]]. The largest rhyolite mass around Castell appears nearly concordant on the map but is discordant at smaller scales. The Moel yr Ogof intrusion forms a neck or plug cutting through the youngest exposed rocks in the Hebog syncline, with explosion [[breccia]]s suggesting a former volcanic vent. Later Caledonian intrusions include microgranophyre sheets along the western side of the Hebog syncline, [[microgranite]]s, [[porphyrite]]s, quartz porphyry [[Dike (geology)|dikes]], and widespread ophitic [[dolerite]]s cutting many faults, indicating late‑stage intrusion during Caledonian [[Deformation (volcanology)|deformation]].<ref name="Shackleton 1961"/>

The distribution of rocks is primarily controlled by [[folding (geology)|folding]], the most important being the Hebog syncline—a continuation of the Snowdon syncline. This fold contains the volcanic rocks at the area's core and is accompanied by other folds such as the Moel Ddu syncline, the [[Llwydmawr]] syncline and the Ynyscynhaiarn anticline. Detailed [[geological mapping]] here has linked previously mapped areas of Snowdon, [[Tremadog]] and [[Nantlle]], greatly enhancing understanding of the Lower Palaeozoic interplay of [[sedimentation]], volcanism and deformation in Britain.<ref name="Shackleton 1961"/>