18/5/2014

The May 2014 fieldtrip was led by Tony Thorp on and around Gaer Fawr Hill near Guilsfield, on the western flank of the Guilsfield anticline. We were near the top of the Gaer Fawr Fm (Ordovician, Caradoc), a shallowing-up sandstone with abundant fossils, as seen in the small quarry at the nature reserve car park. This hard sandstone forms the hill. Above this in the steeply dipping sequence is the softer Dolhir Fm (Ashgill), which has now eroded to a valley. The Ordovician is then topped by the Nod Glas, a thin horizon of soft black shale, which we saw in the stream banks at the valley bottom. Above this in the sequence is the very hard Powis Castle Conglomerate, a beach deposit, now forming a ridge, along which we walked, looking across the eroded valley, at Gaer Fawr Hill. All these features were beautifully evident on a lovely dry day, with additional features to be seen in the stream bed (bentonites, evidence of faulting, and limestone). The day finished with a walk to the top of the iron age hill fort, through woods of stunning bluebell display.