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. On Moel yr Ogof can be found Owain Glyndŵr's Cave.
Stratigraphy The
stratigraphical sequence begins with the
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 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
trilobites 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.
Tremadocian rocks extend into the area along the Ynyscynhaiarn
anticline, with the most fossiliferous exposures showing delicately striped leaden‑grey blocky
mudstones 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
faulted or slipped, though the overall pattern shows Ordovician rocks overstepping Cambrian towards the north‑west, a trend seen throughout
Caernarfonshire. 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
D. geminus.
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
rhyolitic series separated by a basic series. It begins with the distinctive
feldspathic streaks showing characteristic across, basic tuffs and the
Moel yr Ogof Basalts—a sequence of at least five
basaltic lava flows well exposed in cliffs on the south side of Moel yr Ogof. Chemical analyses show these to be distinctly alkaline
synclinal outlier south of the summit of Moel yr Ogof. The area contains numerous
igneous intrusions of two main age groups: those related to the
Caradocian Snowdonian
volcanism and those
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
breccias suggesting a former volcanic vent. Later Caledonian intrusions include microgranophyre sheets along the western side of the Hebog syncline,
microgranites,
porphyrites, quartz porphyry
dikes, and widespread ophitic
dolerites cutting many faults, indicating late‑stage intrusion during Caledonian
deformation. The distribution of rocks is primarily controlled by
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. ==References==