The name is derived from the name of the island which is largely formed from them; these rocks have also previously been referred to as the Manx Slates or Manx Slates Series.
The group comprises dark mudstones with siltstone laminae and some sandstones and which exceed a thickness of 3000m.
It is divided into a lowermost Glen Dhoo Formation which is overlain by (though the entire known boundary is faulted) the Lonan, Mull Hill, Creg Agneash and Maughold formations in ascending order.
A fault separates these from the overlying Barrule, Injebreck, Glen Rushen and Creggan Mooar formations which are in turn separated by a fault from an overlying Ladyport Formation.
[1][2] In stratigraphic order (youngest at top) the formations are: