More recent ice core samples of today's glacial ice substantiated the cycles through studies of ancient pollen deposition. Currently a number of methods are making additional detail possible. Matching the stages to named periods proceeds as new dates are discovered and new regions are explored geologically. The marine isotopic records appear more complete and detailed than any terrestrial equivalents, and have enabled a timeline of glaciation for the Plio-Pleistocene to be identified. It is now believed that changes in the size of the major ice sheets such as the historical Laurentide Ice Sheet of North America are the main factor governing variations in the oxygen isotope ratios.