cap carbonates. In 2010, Francis A. Macdonald, assistant professor at Harvard in the Department of Earth and Planetary Sciences, and others, reported evidence that Rodinia was at equatorial latitude during the Cryogenian period with glacial ice at or below sea level, and that the associated Sturtian glaciation was global. Evidence The snowball Earth hypothesis was originally devised to explain geological evidence for the apparent presence of glaciers at tropical latitudes. According to modelling, an ice–albedo feedback would result in glacial ice rapidly advancing to the equator once the glaciers spread to within 25° to 30° of the equator. Therefore, the presence of glacial deposits within the tropics suggests global ice cover. Critical to an assessment of the validity of the theory, therefore, is an understanding of the reliability and significance of the evidence that led to the belief that ice ever reached the tropics. This evidence must prove three things: that a bed contains sedimentary structures that could have been created only by glacial activity; that the bed lay within the tropics when it was deposited. that glaciers were active at different global locations at the same time, and that no other deposits of the same age are in existence. This last point is very difficult to prove. Before the Ediacaran, the biostratigraphic markers usually used to correlate rocks are absent; therefore there is no way to prove that rocks in different places across the globe were deposited at precisely the same time. The best that can be done is to estimate the age of the rocks using radiometric methods, which are rarely accurate to better than a million years or so. The first two points are often the source of contention on a case-to-case basis. Many glacial features can also be created by non-glacial means, and estimating the approximate latitudes of landmasses even as recently as can be riddled with difficulties. Palaeomagnetism The snowball Earth hypothesis was first posited to explain what were then considered to be glacial deposits near the equator. Since tectonic plates move slowly over time, ascertaining their position at a given point in Earth's long history is not easy. In addition to considerations of how the recognizable landmasses could have fit together, the latitude at which a rock was deposited can be constrained by palaeomagnetism. When sedimentary rocks form, magnetic minerals within them tend to align themselves with Earth's magnetic field. Through the precise measurement