Sandstone concretion

Such bizarre and attractive natural formations, known as concretions, occur in 30 million year old sands left by an ancient river system in what is now the Paris Basin. Although predominately made up of unconsolidated or loose sands, locally ground water rich in dissolved silica has cemented the sand grains to form sandstone. These sandstones were used in the construction of the famous Royal Chateaux de Fontainebleau southeast of Paris. The mineral-rich waters also cemented the sand grains into oddly formed concretions (gogottes); ones with interesting shapes are prized by collectors and museums.

Image Number: ROM2008_10221_19