Ultramarine Glacier is located at the head of Blue Fjord, a glacial estuary that trends generally north for 4.5 miles (7.3 km) to Port Nellie Juan in western Prince William Sound on the east coast of the Kenai Peninsula, about 87 miles (140 km) west of Cordova and 26 miles (42 km) southeast of Whittier, Alaska. The Ultramarine Glacier was named for the clear blue color of the ice. The glacier flows out of the Sargent Icefield and then trends northeast for 2 miles (3.2 km) to its terminus at the head of a 1.4 miles (2.3 km) long proglacial lake. The terminus is presently surrounded by a gravel outwash plain and a fringe of vegetation-free bedrock, which is Jurassic–Cretaceous greywacke of the Valdez Group. A stream of about 1 mile (1.6 km) connects the glacial lake to Blue Fjord. The head of Blue Fjord is an alder-covered outwash fan created by the retreat of Ultramarine Glacier which is the main source of freshwater input to the fjord. The bay was named in 1908 by the U.S. Geological Survey for the color of the water. The rocky western wall of the fjord is near-vertical cliffs whereas the eastern wall is more gently sloping. The intertidal portion of the outwash fan terminates at a slope break at its seaward edge and then descends to the fjord floor with a slope of 17 degrees. The bottom of the fjord at the toe of the outwash plain is at a depth of 65 fathoms (119 m), and slopes to a flat basin with a depth of 104 fathoms (190 m) inside a sill at the mouth of the bay. The sill has a relief of 246 feet (75 m), and outside the sill, the bottom falls away steeply to 280 fathoms (512 m) in Port Nellie Juan. Port Nellie Juan is an embayment that extends generally northeast for 15 miles (24 km) from the mouth of Kings Bay to Prince William Sound in the Chugach National Forest.

Ultramarine Glacier filled Blue Fjord during the Pleistocene which lasted from about 2,588,000 to 11,700 years ago. Several early European explorers visited Prince William Sound but Captain George Vancouver was the first to observe the western shore in 1794. Vancouver sent Lieutenant Joseph Whidbey to survey the shoreline and when he was traveling south from Culross Island he noted a deep embayment to the westward which is represented on Vancouver’s charts by three unnamed arms, two to the southward and one to the west-northwest. In 1887, Samuel Applegate explored the embayment and named it after his schooner Nellie Juan. Applegate represented Ultramarine Glacier on his map as terminating in the sea with an ice cliff facing northeast. In 1908, the glacier was visited by the U.S. Geological Survey and the terminus was on a glacial outwash flat within 0.25 miles (0.4 km) from tidewater and had retreated about 1300 feet (400 m). In 1935, Ultramarine Glacier had receded another 1,000 feet (300 m) since 1908, and another 1000 feet (300 m) by 1964, and 1.2 miles (2 km) between 1964 and 2000. These observations give recession rates of 51 feet (16 m) per year before 1930, 40 feet (12 m) per year from 1930 to 1950, 50 feet (15 m) per year from 1950 to 1957, and 176 feet (54 m) per year from 1964 to 2000. The terminus is presently 2.26 miles (3.6 km) from tidewater, so in the last 134 years since the observations of Applegate, the glacier has retreated an average of about 89 feet (27 m) per year. A series of minor advances and retreats have likely occurred over this period. The most recent advance was documented using a carbon isotope sequence from logs recovered from the head of Blue Fjord.

The dating of glacier recession and intermittent periods of glacier advance is primarily accomplished using dendrochronology. Using this method, the oldest date endpoint for Ultramarine Glacier is represented at the outermost moraine created during the maximum ice advance of the Holocene, where mountain hemlocks as old as 450 years are presently growing. Inside the moraine is a barren landscape of the gravel outwash plain where the glacier terminus has moved back and forth repeatedly killing and burying trees. The youngest endpoint is at the current terminus which is dominated by ice-sculpted bedrock. Mapping of glacial outwash plains often led to the discovery of extensive subfossil forests that were exposed during the ice recession of the twentieth century. Those trees that were buried in glacial till at the location where they once were rooted are considered to give the best estimate for times of glacial advance; however, some stumps preserved in their growth location may have originally been encased in outwash sediments and killed prior to the arrival of the ice margin at that site. Recently deglaciated zones are initially colonized by dense thickets of alder and willow before the successional development of a conifer forest. North Pacific coastal forests fringing the icefields are primarily composed of mountain hemlock together with Sitka spruce and western hemlock. Twenty-nine subfossil logs were found in the outwash plain of Ultramarine Glacier. The oldest log found was dated to be mature in 1306 AD. The earliest reliable kill dates of trees that can be attributed to glacial advance were four trees that were buried between 1692 AD and 1697 AD. Thereafter, a down-valley sequential progression of kill dates occurs with 10 trees overrun between 1701 and 1703, and 12 trees overrun between 1711 and 1715. The remaining three logs were not useable. An additional six subfossil stumps died in their growth location at the Holocene terminal moraine between 1695 and 1766; however, these appear to have been overwhelmed by outwash aggradation and so only constrain the arrival of Ultramarine Glacier at this position after 1766. The dendrochronology suggests that Ultramarine Glacier retreated from its maximum extent between 1880 and 1890, which coincides with the initial observations of Applegate. Read more here and here. Explore more of Blue Fjord here: