| Marros Group | |
|---|---|
| Stratigraphic range: Namurian | |
 Sandstones and interbedded mudstones of the Telpyn Point Sandstone Formation, Haroldston Chins, St Bride's Bay | |
| Type | Group |
| Sub-units | Aberkenfig Formation, Bishopston Formation, Twrch Sandstone Formation, Telpyn Point Sandstone Formation |
| Underlies | Coal Measures Group |
| Overlies | Pembroke Limestone Group |
| Thickness | 20 m to 750 m |
| Lithology | |
| Primary | mudstone sandstone |
| Other | chert, limestone |
| Location | |
| Country | Wales |
| Extent | South Wales Coalfield, Pembrokeshire |
The Marros Group is the name given to a suite of rocks of Namurian age laid down during the Carboniferous Period in South Wales. These rocks were formerly known as the Millstone Grit Series but are now distinguished from the similar but geographically separate rock sequences of the Pennines and Peak District of northern England and northeast Wales by this new name.