Lower House Stream Section

Highlights

Lower House Stream Section is the best and most fossiliferous exposure of the Cancellatum Marine Band in South Wales, and marks the junction between the Marsdenian and Yeadonian stages in this basin.

Introduction

Along this stream, which lies just south of Lower House Reservoir, 3 km north of Rhymney, Mid-Glamorgan, Wales [SO 104 102], is a small exposure in the Middle Shales Formation, as developed on the eastern end of the North Crop of the South Wales Coalfield. Other than a passing mention by Evans (1971), there is no published account of the geology. The following is based partly on an unpublished report submitted in 1978 to the Nature Conservancy Council by T.A. Mackay.

Description

Lithostratigraphy

The exposed sequence is just under 3 m thick. At the top is 75 cm of cross-bedded sandstone with an erosive base. This overlies 2 m of shales, the upper part of which is marine, with a thin limestone band.

Biostratigraphy

The Cancellatum Marine Band here yields a diverse assemblage, including uncrushed ammonoids Cancelloceras cancellatum (Bisat), productid brachiopods, bivalves and crinoid fragments, and marks the base of the Yeadonian Stage.

Interpretation

This is the best exposure of the Cancellatum Marine Band in South Wales. It is known from a number of other localities, such as Tenby–Saundersfoot Coast, Marros Sands, Vale of Neath and Llamarch Dingle (all discussed elsewhere in this chapter), but Lower House yields far better preserved and abundant fossils, especially the 'solid' ammonoids. The marine band is stratigraphically important, in that it marks the boundary between the Marsdenian and Yeadonian stages (see discussion on Orchard Farm in Chapter 2).

Conclusion

Lower House Stream Section is the best exposure in South Wales of marine beds known as the Cancellatum Marine Band. The band is just over 310 million years old, and marks the boundary between two geological ages known as the Marsdenian and Yeadonian.

References