The Marros Group is the name given to a suite of rocks of Namurian age laid down during the Carboniferous Period in South Wales.

The mudstones of these latter two formations was formerly known as the ‘Middle Shales’, a name reflecting the position of this sequence sandwiched between the Basal Grit below and the Farewell Rock, the lowermost sandstone of the South Wales Coal Measures, above.

The sequence as developed along the north crop of the South Wales Coalfield varies along its length but within the mudstone which forms the greater thickness of the Bishopston Mudstone Formation, the following are observed: Note that the names of many of the sandstone units are derived from the marine bands (themselves named for diagnostic fossils within them) beneath which they typically occur.

Faulted blocks of the Twrch Sandstone and Bishopston Mudstone formations are responsible for much of the dramatic scenery within the national park's Waterfall Country.

In contrast, the contact with the overlying Farewell Rock which lies at the base of the South Wales Coal Measures is conformable.