While the valleys that litter the landscape in Glacier National Park belong to the Ice Ages of eons past, the current glaciers that persist today are not nearly that old. Estimates max out at about 5,000 years ago, give or take. Glaciologists know this by looking at the valley formations. A glacier's footprint is pretty easily mapped when you know where it started and ended. All of the current glaciers exist in an existing valley, meaning the Ice Age glaciers fully melted and these smaller glaciers formed inside of the existing footprint of the old. Morraines, or piles of rock deposited by a glacier are those definitive footprints that shows the maximum length and size of a glacier. The morraines of the existing 25 or so still in the Park (what used to be 80) are incredibly small by comparison and reached their maximum in 1850, at the tail end of the Little Ice Age.