It is easy to think that firm evidence for past glaciations lies in sedimentary strata that contain an unusually wide range of grain size, a jumble of different rock types – including some from far-off outcrops – and a dominance of angular fragments: similar to the boulder clay or till on which modern glaciers sit. In fact such evidence, in the absence of other signs, could have formed by a variety of other means. To main a semblance of hesitancy, rocks of that kind are now generally referred to as diamictites in the absence of other evidence that ice masses were involved in their deposition. Among the best is the discovery that diamictites rest on a surface that has been scored by the passage of rock-armoured ice – a striated pavement and, best of all, that the diamictites contain fragments that bear flat surfaces that are also scratched. The Carboniferous to Permian glaciation of the southern continents and India that helped Alfred Wegener to reconstruct the Pangaea supercontinent was proved by the abundant presence of striated pavements. Indeed, it was the striations themselves that helped clinch his revolutionising concept. On the reconstruction they formed a clear radiating pattern away from what was later to be shown by palaeomagnetic data to be the South Pole of those times.
The multiple glacial epochs of the Precambrian that extended to the Equator during Snowball Earth conditions were identified from diamictites that are globally, roughly coeval, along with other evidence for frigid climates. Some of them contain dropstones that puncture the bedding as a result of having fallen through water, which reinforces a glacial origin. However, Archaean and Neoproterozoic striated pavements are almost vanishingly rare. Most of those that have been found are on a scale of only a few square metres. Diamictites have been reported from the latest Neoproterozoic Ediacaran Period, but are thin and not found in all sequences of that age. They are thought to indicate sudden climate changes linked to the hesitant rise of animal life in the run-up to the Cambrian Explosion. One occurrence, for which palaeomagnetic date suggest tropical latitude, is near Pingdingshan in central China above a local unconformity that is exposed on a series of small plateaus (Le Heron, D.P. and 9 others 2019. Bird’s-eye view of an Ediacaran subglacial landscape. Geology, v. 47, p. 705-709; DOI: 10.1130/G46285.1). To get a synoptic view the authors deployed a camera-carrying drone. The images show an irregular surface rather than one that is flat. It is littered with striations and other sub-glacial structures, such as faceting and fluting, together with other features that indicate plastic deformation of the underling sandstone. The structures suggest basal ice abrasion in the presence of subglacial melt water, beneath a southward flowing ice sheet