Porth Neigwl is a 6 km wide, drift-filled embayment, located on the Llyn Peninsula of northwest Wales, UK. The embayment is surrounded by hills formed of Lower Palaeozoic Rocks. Infilling the embayment is a 17 km2 area of thick Quaternary deposits which are well exposed in the sea-cliff. These deposits have been studied intensely over the past century. However, previous workers have made little reference to the total extent or architecture of the Quaternary deposits that lie below present sea level and nothing is known about the rockhead geometry. A programme of high-resolution shallow seismic surveys are described, including 3 km of refraction and 1 km of shallow reflection profiling. Refraction results show that the Quaternary drift cover is variable in composition and thickness with a maximum thickness of 80 ± 8 m. However, results indicate that rockhead within the embayment lies consistently at an elevation of around - 32 m OD. The spatial distribution of refractor velocities is discussed. High-resolution shallow reflection surveys identify three major Quaternary sequences below present sea-level that are consistent across the area. This study demonstrates that combined seismic refraction and reflection studies are very useful in the rapid analysis of Quaternary stratigraphy and spatial depth to rockhead variations in areas of thick drift.

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