The stratigraphy and origin of the portaskaig boulder bed series (Dalradian)

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Abstract
The late Pre‐Cambrian Portaskaig Boulder Bed Series shows a very uniform sequence for some 600 km along the Caledonian strike, from Connemara on the west coast of Ireland to Aberdeenshire in Scotland. The Series, which is divided into four groups of beds, comprises numerous individual boulder beds separated by stratified sediments. The lower boulder beds are carbonate‐bearing and contain abundant intrabasinal fragments of dolomite and few extrabasinal crystalline rocks while the higher beds are psammitic and contain mainly extrabasinal crystalline rocks. Varve‐like banding, involutions and sandstone polygons suggest periglacial conditions. It is concluded that the boulder beds represent subaqueous tills deposited on unconsolidated sediments. Contemporaneous deformation of some of these sediments is attributed to the movement of ice, although the possibility that sliding of the unconsolidated sediments occurred locally is considered.

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