At the beginning of the Cryogenian Period, around 717 million years ago, ice caps advanced abruptly to cover the entire planet. Global ice cover is thought to have been attained relatively quickly, over thousands of years, because of the albedo effect – that is, the more ice there is, the more sunlight is reflected back into space, and vice versa. After reaching the equator, ice sheets persisted there for over the next 80 million years. Understanding how our planet could have plunged into such extreme cold is still the subject of intense debate, but there is a problem. Sedimentary records of the moment when ice first reached low latitudes are absent everywhere on Earth, or so we once thought.

Off the west coast of Scotland lies a chain of tiny islands called the Garvellachs that may hold some of the clues to a time in earth’s history known evocatively as ‘Snowball Earth’. The islands host > 1km of glacial rocks deposited between 717 and 660 million years ago, marking the first of two global glaciations in the Cryogenian. Perhaps more intriguingly, the Garvellachs also preserve a demonstrably transitional record from underlying carbonate rocks of the Tonian period into glacial rocks of the Cryogenian. In terms of climate, this can be interpreted as warm tropical seas, full of bacterial life, gradationally transitioning into a frigid climate, before definitive ice caps moved over the land; akin to seeing Greenland-scale ice caps descending on the Caribbean today. Elsewhere on Earth this pivotal transition into the Cryogenian period appears to have been removed by the erosive power of glaciation, resulting in a time gap between sedimentary layers. However, on the Garvellachs no such gap is found, making it a truly unique location globally for Snowball Earth science. These Scottish islands may prove integral for understanding how and why Earth descended into a Snowball Earth climate state whilst having ramifications for defining the geological timescale.