Not only the Harbours near Rotterdam are built with bolders and stones from abroad. Also the dykes upstream of the Maas and Ijssel are based on material from mostly Germany and Belgium. When examining carefully, sometimes nice finds are possible. The pictures shown are finds from the area of Hollandse Ijssel, Lek (Zaag) and Noord. A few (like the first one) originate from Slag, others from Porfier. Two are biological Induced mineralization (BIM).