Hjarnø is a small Danish island at the mouth of Horsens Fjord on the east coast of Jutland in Hedensted Municipality.

[10] Along the coast of Hjarnø, archaeologists have discovered several finds that indicate previous coastline settlement by people of the Mesolithic Stone Age.

[11] Due to changes in the shoreline caused by the end of the Ice Age, many of these settlements were submerged and therefore preserved.

Although the relieved weight of the melted ice has led to most of Denmark’s shores rising, around Horsens Fjord, which sits in an area that is sinking due to fracture lines in the earth’s plates, the coastline is instead falling.

[9] This depression of the coastline has resulted in the trapping of many artifacts in an anaerobic environment, called the gyttja, which led to their preservation through the centuries.

[11] In the past few decades, erosion along the seabed has been exposing previously covered Stone Age sites, a process that is evident at Hjarnø.

[11] In 2008, Peter Astrup, of the University of Aarhus, during a survey of a previously known submerged Stone Age site along the coast of Hjarnø, noticed that erosion had exposed many layers of the gyttja.

[11] There were also numerous stakes made of hazel wood, hazelnut shells, plant seeds, string, and flint flakes and tools.

The site was known to the 12th/13th-century Danish historian, Saxo Grammaticus, who recorded a legend about its origins in his Gesta Danorum.

[6] However, over time, residents of Hjarnø removed stones for other purposes, for example, the Kriger Monument in Fredericia.