Tillite lithified by magnesium silicate hydrate (M-S-H) cement has been formed at the surface of the Feragen Ultramafic Body in SE Norway. Serpentinization of olivine-rich rocks led to formation of brucite, which dissolves during weathering to form a high-pH (>9) Mg-rich fluid. This fluid dissolves quartz deposited by glaciers during the Weichselian glaciation. Subsequent evaporation leads to the precipitation of a nanocrystalline magnesium silicate hydrate phase with the approximate composition Mg8Si8O20(OH)8·6H2O. The resulting cemented rock is characterized by disintegrated and partly dissolved quartz grains that are surrounded by the M-S-H cement. This process occurs at surface conditions in a subarctic climate and is known to take place on a timescale of a few decades, as constrained by mining activities. Magnesium-based cement is currently of high interest as the search for environmentally friendly cement is a pressing task considering that the widely used Portland cement accounts for about 7% of the worldwide anthropogenic CO2 emission. As this study provides new key insights into the formation process of natural CO2-neutral magnesium-based cement on the field- and microscale, it could contribute in the development of a new sustainable construction material.