Geology during the Ice Age

The Mossfells originally started emerging from of the North Atlantic during the early Miocene 22.1 million years ago. During this time period, global temperatures and sea levels were much higher than present. From the middle to late Miocene, the first islands in the chain were likely covered in thick tropical forests, low-lying mangroves, and savannahs. Taller areas would be have dry mountain climates with extremely dry rain shadow deserts. Fossil analysis on Nishsystir and the Lonely Bob seamount supports this hypothesis through the presence of tropical flora and fauna fossils predominating this period. During this period, the islands were largely separated from the mainland of North America, leading to the development of several of the oldest linages of birds, amphibians, reptiles, and endemic flora on the islands

However, like many locations in the Northern Hemisphere, the Mossfells have also been shaped by the the tremendous power of glaciation and sea level change. Following the Mid-Miocene disruption, global temperatures cooled. With the drop in temperatures came a precipitous drop in sea levels. During this transition, the Mossfell's climate began to change quickly, favoring deciduous and coniferous forests instead of tropical ones. Colder winters led to the development of a dry steppe on the peaks. Around nine million years ago, the newly rising islands of Neffannafjall and Gasmensyi likely had snowy peaks until mid-spring. Throughout this cooling climate, many new species were able to raft over to the southwestern-most islands. This lead to the introduction of many new species - particularly a new wave of mammals, fowl, and cold-tolerant reptiles.

As the climate continued to cool through the Pliocene and into the Pleistocene, several island were connected by land bridges. This facilitated the exchange of flora and fauna across the islands. Both the change of climate and the introduction of species across islands proved to be a destructive force for many older linages. While the island was mostly buffered from the climatic extremes of the continent, the change in temperatures led to a minor extinction event on the islands as several species of flightless birds and reptiles perished in an increasingly cooler world. The cooling climate also led to the introduction of animals from colder climates. Fossil evidence indicates the presence of walrus and seal colonies on the islands starting five million years ago. The presence of the Jefferson's Eagle-Owl (Bubo insulagard) , an endemic relative of the snowy owl, also provides evidence that arctic birds began to colonize the islands.

Due to the island's distance from the North American mainland, and the warming (albeit feeble) influence of the gulf current, the islands never were enveloped by the Laurentide Ice Sheet. This flora and fauna of the islands were spared extinction via glaciation. Despite this, glaciers were still present in great quantities during this period, especially on Neffannafjall and Mossfellheim. The Neffannafjall inlet is a great example of glacial erosion. The depression between volcanic peaks was already present prior to glaciation, however the erosive forces aided in giving Neffannafjall it's distinct cracked-bowl shape.

Eventually, following the passage of the Last Glacial Maximum 20,000 years before present, sea levels around the islands began to drop. This led to land bridges once again being subsumed by the Atlantic Ocean. Some land bridges would persist until the end of the Younger Dryas period, particularly the Neffannafjall-Mossfellheim and St. James-St. Anne land bridges. This island separation, and a warmer more fruitful climate, once again led to a brief period of speciation before initial human contact with the island around 11,000 years ago.