Editing Moel Hebog (section)

===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"/>