Summary of site:
The deep coastal indentation which is Portballintrae is developed along a major north/south trending fault raked out and widened by marine erosion. The land surface around the bay has been scoured by glaciers and local striations on bedrock tell that the final phase of ice movement was northwards. The glacial and immediate post-glacial deposits which form the main interest of the area occupy an undulation of the bedrock on the west side of the bay.
Three associations of sediment (facies) can be seen at the site. The lowest is a very thick muddy paste (a diamict) containing a wide spread of particle sizes and is divided into crude beds separated either by sands or by changes in texture. The larger particles are mainly basalt, the rock underlying the area.
Immediately above the diamict is a rhythmite, a sequence of rocks consisting of regularly repeated “rhythms” of sediment types. In this case each commences with fine laminations, (sometimes with cross bedding) of mud with streaks of silt and sand, followed by rippled sands with mud streaks and thin muds lying in the ripple troughs, succeeded by erosive ripples showing a variety of structures. These final ripples are draped with fine mud laminations which complete each unit before the sequence is repeated. There are 14 identifiable rhythms in the succession.
The rhythmite is abruptly terminated and topped by gravels and sands which scour it and fill the created hollows. These gravels and sands have complex bedding, many sudden changes of texture and their particles are angular for the most part.
These deposits reveal a well marked series of events. The diamicts are debris washed from the aprons of sediment surrounding retreating ice sheets. The overlying rhythmite results from a sequence of at least 14 storms. This is clear evidence that the ocean margin lapped the shore, each storm repeating a pattern of activity ending with the draping of the sea bed as suspended fine mud gently settled. Cold water microfossils, largely of two species, are present in the muds indicating that the events were immediately post-glacial before the progressive warming of the sea had commenced.
The gravels and sands that abruptly terminate the rhythmite show the start of marine erosion a response to falling sea level as the land rose to compensate for the loss of its ice load.
This combination of sediments is unique in the north west British Isles because in normal circumstances it would have been completely destroyed by erosion during uplift. Here it is preserved by some quirk of local geography. The site is important because it provides evidence of the scale, timing and local conditions during the last stage of glacial retreat, key elements for any future reconstructions of events.