structural:summarization
Geology

According to Hole, "The geology of Alexander Island can be attributed mainly to processes associated with the subduction of proto-Pacific oceanic crust along the western margin of the Antarctic Peninsula, from latest Triassic to Late Tertiary times." The LeMay Group accretionary prism complex, along with plutonic and volcanic rocks, are prevalent along the western portion of the island. The LeMay Group consists of variably-deformed and metamorphosed sedimentary and igneous rocks. Although it is dominated by deformed arkosic sedimentary rocks, it includes turbiditic greywackes, black mudstones, and conglomerates. The 4 kilometers (2.5 miles) thick Upper Jurassic to Lower Cretaceous Fossil Bluff Group sedimentary rocks outcrop as a 250 kilometers (160 miles) long by 30 kilometers (19 miles) wide belt along the eastern coast. This Fossil Bluff Group consists of a basal deep-marine assemblage 2,200 meters (7,200 feet) thick, overlain by a mudstone assemblage up to 950 meters (3,120 feet) thick, followed by a shallow-marine assemblage of coarsening upward sandstones. Alkali basalts erupted after the cessation of subduction. These range in age from the tephrites at Mount Pinafore (5.5–7.6 Ma), to the basanites at Rothschild Island (5.5 Ma) and Hornpipe Heights (2.5 Ma), to the alkali and olivine basalts on Beethoven Peninsula (<1-2.5 Ma).

The LeMay Range Fault trends N-S, parallel to the George VI Sound, and the Fossil Bluff Formation is downfaulted to the east of this fault against the LeMay Group. Sand dykes are found against this fault zone and in many other parts of the Fossil Bay Formation. Fossils within the Fossil Bluff Formation include ammonites, belemnites, bivalves, and serpulids.