Northwest of St. Mary from Many Glacier in Glacier National Park, the trip to Grinnell Glacier combines hiking with two boat trips passing a total of 37 glaciers, snow bridges, crevasses and ice caves. Since there can be long lines for the boats, the best best is to be on the first one from Many Glacier across Swiftcurrent Lake (typically 8:30am). At the south end of Swiftcurrent Lake, hike over to Lake Josephine and take a second boat to the southwest shore. Then, climb the switchbacks to Grinnell Lake and Grinnell Falls. Past the steep cliffs above Grinnell Falls is a picnic area at the base of a moraine. A quick scramble atop the moraine leads to the fissures and ice cave in Grinnell Glacier. Return via the boat (last from Lake Josephine at 4:40pm) or hike the shoreline trails.
Overall, Grinnell Glacier is the 13th most popular hiking trail of all 19 hikes in Montana.