Tree-ring studies indicate that retreat of the recent glaciation began about 1850. When Glacier National Park was established in 1910, there were more than 150 glaciers within the national park compared to about one fourth of that number now. Retreat rates appear to have been slow until about 1910. There was a period of rapid retreat during the mid- to late 1920s. This corresponds to a period of warmer summer temperatures and decreased precipitation in this region. Several of the larger glaciers separated into two smaller glaciers at this time. The Jackson and Blackfoot Glaciers separated as did the Grinnell and Salamander Glaciers.