The Usk Inlier is a domed

Dome (geology)

In structural geology, a dome is a deformational feature consisting of symmetrically-dipping anticlines; their general outline on a geologic map is circular or oval...

 outcrop of rock strata of Silurian

Silurian

The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 age in Monmouthshire

Monmouthshire

Monmouthshire is a county in south east Wales. The name derives from the historic county of Monmouthshire which covered a much larger area. The largest town is Abergavenny. There are many castles in Monmouthshire .-Historic county:...

 in south-eastern Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

. It is located in the countryside between the towns of Caerleon

Caerleon

Caerleon is a suburban village and community, situated on the River Usk in the northern outskirts of the city of Newport, South Wales. Caerleon is a site of archaeological importance, being the site of a notable Roman legionary fortress, Isca Augusta, and an Iron Age hill fort...

 and Pontypool

Pontypool

Pontypool is a town of approximately 36,000 people in the county borough of Torfaen, within the historic boundaries of Monmouthshire in South Wales....

 and the village of Raglan

Raglan, Monmouthshire

Raglan is a village in Monmouthshire, south east Wales, United Kingdom. It is located some 9 miles south-west of Monmouth, midway between Monmouth and Abergavenny on the A40 road very near to the junction with the A449 road...

. The longer axis of the dome or 'pericline', often referred to as the Usk Anticline, is aligned north-south. The inlier is largely surrounded by a sequence of Old Red Sandstone

Old Red Sandstone

The Old Red Sandstone is a British rock formation of considerable importance to early paleontology. For convenience the short version of the term, 'ORS' is often used in literature on the subject.-Sedimentology:...

 rocks of Devonian

Devonian

The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

age, though both these and the Silurian rocks are largely obscured by superficial deposits.