Glacial deposits had been found in sites from rocks that dated to the Late Proterozoic, but were these sites located at the equator? Brian Harland had first used paleomagnetic data that seemed to show that a number of Late Proterozoic glacial deposits were formed near the equator. But much of this paleomagnetic data was suspect as new techniques for collecting, measuring, and calculating paleomagnetic data improved since the 1960s. Kirschvink revisited many of these sites and tested the rocks to select only the most favorable natural remnant magnetization (NRM) from the rocks. This data was collected to remove the possibility that the rocks had been re-magnetized, which would have realigned their magnetic minerals since being deposited. Paleomagnetism measures the orientation of magnetic minerals in rocks. When the magnetic minerals are measured and compared based on the exact position, inclination and orientation of the source rock geologists can determine where in respect to the magnetic pole the rock was deposited. A vertical orientation shows the rock was deposited at the magnetic poles, while a horizontal orientation shows the rock was deposited at the magnetic equator. Using paleomagnetic data collected from dozens of locations across the globe Kirschvink determined that the glacial debris had indeed been deposited near the equator.
Other evidence for the global ice age was the presence of banded-iron formations found during this same time period. Banded-iron formations (BIFs) are common from very early in Earth's history (before 1.8 billion years) and are believed to have formed when the Earth had little free oxygen in the atmosphere. Iron is found in seawater and it is believed that the early oceans had abundant iron that precipitated out when it encountered oxidizing surface waters. Once oxygen levels in the atmosphere rose to higher levels, BIFs were no longer able to form, however they were seen again in the Late Proterozoic. Kirschvink again thought that during a global glaciation lasting millions of years the gas exchange between ocean and atmosphere would be reduced and the deep oceans would become anoxic (without oxygen). This would allow iron to build to high concentrations and once the glaciation ended the iron would precipitate out.
