Sumdum Glacier begins on the south flank of Mount Sumdum, whose summit reaches 6,666 feet (2,032 m), and flows southwest to a hanging terminus at 2,100 feet (640 m), where Powers Creek starts its 2-mile (3.2 km) run to the eastern shore of Endicott Arm, about 89 miles (144 km) northeast of Sitka and 50 miles (81 km) southeast of Juneau, Alaska. The name was first published in 1892 by the US Coast and Geodetic Survey as “Soundon”; the Tlingit name reputedly refers to the booming sound of calving icebergs. Powers Creek was named in 1906 by Arthur C. Spencer and Charles W. Wright after a local prospector working a small gold placer there. Over the last 200 years, Sumdum Glacier has retreated from Endicott Arm and now terminates on a steep slope as a hanging glacier, losing mass principally through icefalls and avalanches. The glacier descends nearly 5,000 feet (1,524 m) over 2.8 miles (4.5 km) to an elevation where more temperate conditions destabilize the ice-bedrock interface. As the glacier slides over bedrock irregularities, cavities form beneath the ice, adhesion is lost, and the ice sheet fails catastrophically in an icefall. Sumdum Glacier also has a small accumulation zone above the firn line, where snow gain exceeds losses from melting, evaporation, and sublimation; the steep slope gives accumulated snow little residence time at higher elevations, driving gradual thinning and upslope retreat.

The glacially striated cliffs of Endicott Arm and Tracy Arm expose the granite and partially metamorphosed rock of the Coast Range Batholith. Mountain building in the Coast Range began 115 million years ago when a chain of volcanic islands, riding an oceanic plate, collided with the western edge of the continental plate in the northeast Pacific. The islands were welded to the continent by molten rock that cooled deep underground to form the batholith—the largest body of granitic rock in North America. The western metamorphic belt forms the batholith’s western margin throughout most of southeastern Alaska. Between Tracy Arm and Endicott Arm, ore-bearing outcrops occur in a northwest-trending belt of mineralized metamorphic rocks; the best-known mines and prospects, at Point Astley and Sanford Cove, targeted gold around 1900 and also yielded significant silver as a byproduct. After that early boom, activity quieted, though a titaniferous magnetite deposit was found at Port Snettisham in 1918 and a zinc-copper prospect in Tracy Arm in 1916.

The Sumdum Glacier mineral belt extends about 32 miles (51 km) along the southwest side of the Coast Range batholithic complex and contains three significant mineralized areas: the Tracy Arm zinc-copper prospect, the Sweetheart Ridge gold-copper occurrence, and the Sumdum copper-zinc prospect. The last was discovered in 1958 by the Alaska Helicopter Syndicate in rugged terrain on Mount Sumdum, at elevations ranging from 2,000 feet (610 m) to the 6,666-foot (2,032 m) summit. The prospect lies within a few thousand feet of the western margin of the Coast Range batholith, in contact-metamorphosed rocks. Ore occurs in northwest-trending zones that are generally untraceable beyond a few hundred feet owing to snow cover and the glacier itself; however, diamond-drilling data show that at least two zones are continuous for 10,000 feet (3,048 m) and range from 1 to 50 feet (0.3 to 15 m) thick. Pyrite and pyrrhotite are the dominant sulfide minerals, with chalcopyrite and sphalerite as secondary minerals; minor bornite, chalcocite, malachite, azurite, and galena complete the ore assemblage. Read more here and here. Explore more of Sumdum Glacier and Powers Creek here: