Causeway

Cadw's description

Location

Located off the B 4418 to the north of Dorothea House, beginning beyond Pont Wythdir and running north-east for approximately 100m across marshy ground with a spoil heap to the north-west on the southern edge of Dorothea Quarry.

History

The Nantlle Vale has a long and important history of slate quarrying dating from Roman times. The Dorothea Quarry was a large open-cast pit formed out of 6 small pits, the earliest of which was sunk c1820. The causeway was probably constructed sometime between 1805 and 1830 as part of a road laid to transport slate from the Tan-yr-allt Quarry (opened in 1805), and perhaps others on the south side of the Nantlle Vale, up to the main east-west road near Talysarn Quarry (opened in 1790) and the Nantlle Railway (opened in 1828). This road originally passed the western edge of the Dorothea Quarry but it was cut at the north-east end following a C20 cliff fall at the South Dorothea Quarry (opened in 1860) and so it remains in its earlier C19 form.

Exterior

Causeway constructed of slate rubblestone with some boulders to the base and quoins; flat coping. Both sides have central segmental arch with flanking tapered pilasters together with voussoirs and arch-ring set below a string course which continues either side over square-headed and slate-roofed culverts, 2 to south-west end and 3 to north-east; the former are still open all the way through; rubble soffit to central arch. The parapet is beween 1.2m and 1.5m high and the width is approximately 5m. The road level climbs towards the north-east end where the parapet has partly collapsed.

Reason for designation

Listed for its special interest to the history of the Welsh slate industry as a rare example of an unaltered early C19 slate road at a particularly important quarry.