The falls occur where a small river, the
Nant Llech (), drops over the
faulted edge of a hard sandstone known as the
Farewell Rock which forms the top half of the rock face and the base of the South Wales Coal Measures. Beneath this, and forming much of the recessed portion of the drop, is the Subcrenatum Sandstone separated from the Farewell Rock above by the
Subcrenatum marine band. Both the marine band and sandstone are part of the Bishopston Mudstone Formation included within the
Marros Group, the modern name in South Wales for the assemblage of strata that was traditionally known as the Millstone Grit series. A stream gully between the descent path and the falls marks the line of the Henrhyd Fault which is responsible for the falls' presence. One of the area's most famous visitors was Sir
William Edmond Logan (later head of the
Geological Survey of Canada). In 1833 he carried out detailed geological survey work in the area, discovering a mile (1.5 km) down the valley from the foot of the falls the
fossil trees that now stand in the garden at
Swansea Museum. They are 'Object 1' in their 'History of Swansea in 20 Objects'. ==Access==