Chapter 1: The Port Askaig Formation – a celebrated ‘tillite’: its place in Neoproterozoic science and the history of its study

The Port Askaig Formation (PAF) is a leading example of the deposits of ‘Sturtian' panglaciation. It was first documented in 1819 by MacCulloch, and in 1871, via the work of Thomson, it became the first Precambrian deposit recognized to have a glacial origin. In the early 20 ^th century, Geological Survey staff and others focused on understanding the Dalradian geological succession of SW Scotland in which the Port Askaig rocks constituted an important marker horizon. The second half of the 20 ^th century saw an intense phase of study culminating in important work by Spencer and Howarth that established a stratigraphy of five members correlatable across the NE-trending outcrop in Scotland and Ireland with an archive of depositional events unmatched globally. Spencer’s generalized model of repeated subglacial deposition followed by exposure, periglacial action and marine submergence was challenged by Eyles, Eyles and Arnaud but supported by Benn and later by Ali, Spencer and others in the first results of the current phase of study. Chemostratigraphic works by Shields, Prave and Sawaki, and sedimentological work in the current project led to the suggestion in 2018 that the Garvellachs outcrops were a candidate for a global stratotype of the base of the Cryogenian System.