Unga Island is part of the Shumagin Islands, situated south of the Alaska Peninsula in the Gulf of Alaska, about 64 miles (103 km) north-east of King Cove and 11 miles (18 km) northwest of Sand Point, Alaska. It is the site of a petrified forest exposed along the eroding northern shore. The island name derives from the Unangan Aleut language and was first reported as “Ounga” in 1827 by Admiral Adam J. von Krusenstern of the Imperil Russian Navy, and as “Unga” in 1840 by Ivan P.Veniaminov, a priest with the Russian Orthodox Church. Unga Island is the largest of 20 islands in the Shumagin archipelago. Other large islands include Popof, Korovin, and Nagai. Smaller islands include Andronica, Big Koniuji, Little Koniuji, Simeonof, Chernabura, and Bird. The archipelago was discovered during the Great Northern Expedition led by Vitus Bering in 1741 aboard the Saint Peter. The expedition stopped to fill barrels with water from a brackish pool near the beach. Georg W. Steller, the expedition’s physician and naturalist, knew this water was unsuitable and protested strongly against taking it aboard, warning that its use would lead to a rapid increase in scurvy. Ten men soon fell sick with scurvy and were taken ashore for rest and fresh air, but Nikita Shumagin died soon after landing and was buried on Nagai Island. In his memory the island was named Shumagin, a name now applied to the entire archipelago.

The Alaska Peninsula and the Shumagin Islands are part of the Aleutian Range volcanic belt, which extends 1,000 miles (1,610km) from the west side of Cook Inlet to the western end of the Aleutian Island chain. Unga Island formed as a result of volcanic activity, although no active volcanoes are present today. The eastern half of the island is mountainous and consists largely of andesitic and dacitic lava flows. Fragmental rocks resulting from explosive volcanic activity are common and include coarse volcanic breccia and fine-grained tuffs. The western half of the island is low, with rolling hills and grassland covering rocks of the Unga Conglomerate, a sedimentary rock formation several hundred feet thick that buries the volcanic rocks. The deeper part of the formation consists of thin layers of sandstone, conglomerate, shale and lignite that developed during the Miocene epoch, from 25m to 11m years ago. The upper portion consists of coarse fragments of volcanic breccia with embedded petrified trees. The topography of the entire island has been affected by glaciers, which scoured the surface during the last ice age. Unga Island has risen about 100 feet (31 m) since the Pleistocene, and the lower, flatter parts of the island are old marine terraces that formed when sea levels were lower and the outer coast of the Shumagin Islands was the south shore of the Alaska Peninsula. Archaeological investigations and radiocarbon dating of artifacts collected from these ancient terraces suggest that Aleut people came from the south and arrived about 4,000 years ago. Today almost the entire population of the Shumagin Islands lives in the city of Sand Point on Popof Island.

Petrified logs and stumps are exposed mostly in the lower intertidal and subtidal zones along about five miles (8km) of the northern shore of Unga Island, between Unga Spit to the east and Bay Point to the west. The stumps range in diameter from two feet (0.6 m) to about nine feet (2.7 m), and the longest log is 59 feet (18 m). These are remnants of a Sequoia or Metasequoia forest buried by sudden deposition of volcanic breccia derived from nearby volcanic eruptions during the early Cenozoic era, about 65 million to 2 million years ago. The trees were subsequently preserved by permineralization, in which groundwater dissolves silica from volcanic rocks, fills the pore spaces in the wood, and the silica is remineralized. Silicification also replaces the organic cellular tissue so thoroughly that many silicified trees consist of close to 100% silica, preserving the smallest and most delicate structural details of the wood cells. Any substrate that is able to bring elevated silica into solution is also capable of silicifying plant material. This can be volcanic tephra rich in glassy material or a silica-rich hydrothermal solution from a hot spring. Plant tissue apparently has the ability to attract silica from aqueous solutions and precipitate the mineral in the cells as opal and chalcedony. Without this ability, wood and all other plant tissues would rapidly decompose in an oxygenated environment, and this may be the single most important process involved in the preservation of plants in the fossil record, which allows insight into past ecological and climatic conditions on ancient land surfaces. The Unga Island petrified forest is part of the Alaska Maritime National Wildlife Refuge and has been designated a National Natural Landmark. Read more here and here. Explore more of Unga Strait and the Shumagin Islands here: