Summary of site:
The rocks at Dog Leap are of Precambrian age, perhaps around 590 million years old, belonging to the Ballykelly Formation (part of the Southern Highland Group). They predate obvious fossils.
As might be expected with rocks of this age, they have been metamorphosed i.e. altered by heat and pressure, while deeply buried. The minerals chlorite, epidote, muscovite, quartz and albite, which are common throughout the Ballykelly formation, are all present and indicate a relatively low degree of metamorphism (‘Greenschist facies’).
The common rocks of the formation are psammites (rich in quartz) and pelitic (of shale-like composition) schists but there are also slates, thin limestones and epidiorites (rocks with basaltic chemistry).
All the rocks have complex structures caused by plastic deformation, with new minerals growing during compression, folding, faulting and extensive jointing. There have been several phases of cleavage and foliation, each giving the rocks a distinct ‘grain’ by aligning minerals, and often creating planes of weakness in the process.
Metamorphic rocks are notoriously difficult to interpret but the Dog Leap succession suggests a series of turbid sediments created by the sliding and mixing of sands and muds on the continental flanks of a large ocean basin, far from land. Volcanicity appears to have been associated with their formation (indicated by the epidiorites). The thin limestones are probably biological remains - but of what it is now impossible to say. The rocks of the Claudy Formation, immediately below the Ballykelly, indicate shallow water environments, so a progressive deepening of the sea floor seems to be a major trend in late Dalradian times.
Long after the close of the Precambrian, during the Ordovician period (from around 450 to 475 million years ago) the rocks were metamorphosed and intensely deformed, giving rise to the many structures evident at Dog Leap. This phase of mountain building is part of the Caledonian orogeny.
Dog Leap is regionally important because it is the best exposure showing the rock types and structures of the Ballykelly Formation and has the added advantage of public ownership (by its inclusion in the Roe Valley Country Park). The water-polished rocks in the river bed clearly show the detailed structures of the formation and access is assured and safe.