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The Northumberland Formation (previously also known as the Lambert Formation) is a Late Cretaceous geological formation exposed in the Hornby, Denman, Mayne, and Gabriola Islands, all located near Vancouver Island in British Columbia, Canada; it belongs to the larger Nanaimo Group. Dated to the upper Campanian, numerous vertebrate, invertebrate, and plant fossils have been recovered from the formation, including birds, pterosaurs, sharks, coleoids, ammonites, angiosperms, and gymnosperms.

Geology
The Northumberland Formation is exposed in small strips of the Gabriola and Mayne Islands, east half of Denman Island, and along the western to southeastern shores of Hornby Island. All of these islands are situated east of Vancouver Island in the Strait of Georgia. Certain areas of Hornby Island that expose the Northumberland Formation were formerly mapped as the defunct Lambert Formation and the Spray Formation, being reassigned with evolving interpretations and nomenclature. Part of the Nanaimo Group, the Northumberland Formation overlies the De Courcy Formation and underlies the nonfossiliferous Geoffrey Formation, both dominated by thick-bedded sandstone and conglomerate.

Paleoenvironment
Various interpretations of the Northumberland Formation paleoenvironment have been proposed. A deep-water submarine fan complex has been put forth by several authors based on lithology. Contrasting interpretations argue that the Northumberland Formation was deposited in a shallower environment. Jenkins et al. (2017) suggest an middle to outer shelf 100-300 m in depth based on macrofossil and lithological evidence, supported by foraminiferal data.

Sharks
All following information follows Cappetta, Morrison, & Adnet (2021), unless stated otherwise.