Triassic mudstone. Blue Anchor bay.

The specimen was collected from debris on the beach at Blue Anchor bay on the Somerset coast. The occasion was a field trip organised by the club in June 2014. The mudstones here are rich in nodules of pink alabaster ( fine grained gypsum ) hydrous calcium sulphate, a secondary product of the original Triassic evaporites which were deposited in the first instance as halites from drying up salty playa lakes. The beach was littered with quite a number of large boulders, some as large as 2 metres that were detached from the cliff by erosion, and with the same characteristics as the specimen.  

Tonalite, from Coed-y-Brenin.

The name tonalite is derived from a type locality rock, adjacent to the Tonale line, which is a major structural lineament in the Alps. This specimen was collected on a field trip to Coed-y-Brenin in the Snowdonia national park, North wales. Tonalite has the same petrological significance as andesite, but has remained at depth, crystallising under plutonic conditions, allowing crystals to grow to a reasonable size, and evolving chemically in a felsic sense. Tonalite is composed of essential quartz, Plagioclase feldspar, mafic minerals ( biotite, pyroxene, hornblende ), and minor alkali feldspar. The inserts show crystals of pargasite, which have been exposed on a fractured surface.  A full account of the field trip to Coed-y-Brenin  in November 2020 can be found on this website under the heading of past news.

Septarian concretion (nodule)

Concretions are very common in the mudstones of the Welsh basin. Most of them are found without an apparent mineral content. There are exceptions with an occasional calcite core. This specimen, which has an infilling of calcite is the only septarian concretion that I have observed in central Wales. It was discovered at Abermule, very near to the river Severn, 6Km NE of Newtown. This area is on the outer margins of the Welsh basin, and septarian concretions are known to form at relatively shallow depths of water and sediment.

Flute casts

This specimen shows flute casts on both sides. It is an unusually thin turbidite, being only about three mm. thick. The specimen is sandy- silty rather than muddy, which suggests that it is a proximal turbidite. It was collected from a small quarry just on the outskirts of Dolfor village, near Newtown, Powys. Between Dolfor and Llandrindod Wells. This quarry has yeilded some interesting specimens, including several cm. of bentonite, some quite large loaf shaped concretions, and concretions with about 50% interior calcite.

Chatwell sandstone. Onny valley.

This specimen of Chatwell sandstone was collected on the Onny valley trail, which is on the outskirts of the town of Craven arms, in Shropshire. This is from the higher level of the sandstone, is green in colour,  and packed with numerous shells. The sandstone is in the Caradoc series of the Ordovician period.