 Recent glacial episodes included the Bull Lake Glaciation that began about 150,000 years ago and the Pinedale Glaciation that probably remained at full glaciation until 15,000–20,000 years ago.Ninety percent of Yellowstone National Park was covered by ice during the Pinedale Glaciation.The little ice age was a period of glacial advance that lasted a few centuries from about 1550 to 1860.