Dolomitic Conglomerate is all things to all men (and women)
The term dolomitic conglomerate refers to all of the Permotriassic deposits around the flanks of the Mendips and the limestone outcrops furthur north
As mentioned in previous posts, the DCg was deposited in a desert environment and is basically a Conglomerate or Breccia composed mainly of Carboniferous Limestone, but there are also some outcrops of Conglomerate / Breccia with Old Red Sandston clasts (the geological name for the stones in a conglomerate) The red stuff that stick the clasts together (the matrix) is marl, and is similar/identical to the red marls that make up the Mercia Mudstone Group MMG (formerly called the Keuper Marl).
Marl is a lime clay and is almost certainly derived from denudation / weathering of the Carboniferous Limestone.
The clasts are derived from the surrounding high ground (Mendip hills) and represent alluvial outwash fans found at the bottom of gorges and valleys. Most of the Mendip villages are built upon these outwash fans as they provided relatively level ground above the level of the surrounding marshland. Good examples are Cheddar, Draycott, Westbury, Burrington, Blagdon, Compton Martin, etc.
The Consistency of the DCg is variable and it can be observed in various locations as a conglomerate, a breccia, dolomitised, not dolomitised, limestone clasts, sandstone clasts, etc.
The name was given, as are all geological names, from the location that the rock was first described, or from the first description.
The dolomitisation is very local and only in a small number of outcrops. Principally the rock is composed of limestone clasts in a marl (limestone) matrix and is therefore almost 100% limestone and soluble. It is also jointed so caves will, and have formed in it. All of Wookey Hole from the entrance to 9 is in DCg as are Wigmore Swallet and Red Quar (Attaborough Swallet).
The dolomitisation is only found at the bottom of the alluvial fans and is thought to be related to a much more recent thermal event of rising mineral rich waters that emplaced nodules of Celestine into the marl, It is probabally the same event that caused the mineralisation of the Mendips (Lead, Zinc, etc.)
The Clasts were formed, as has been suggested above, by freeze-thaw action in a desert environment as small stones split from the sides of the valleys and gorges. Typical desert erosion, very hot daytime and below freezing at night. This formed scree slopes down the sides of the valleys, These stones start angular and any that remained in situ stay angular and when cemented by the Marl became breccias but others were transported down the valleys were rounded off and became conglomerates. The transport system was flash floods and if you look at deposits of DCg you will see it is layered/bedded between 200mm to 1 metre thick, each layer represents a single flash flood
Hope this helps
