Calcareous sandstone?
Luxembourg’s solid geology is thickly mantled by deposits of cover-sands laid down when the Baltic ice sheets melted and sent millions of gallons of meltwaters to the North Sea Basin. Locally, those loose sands could be consolidated when mineral-rich springs bonded the grains into a tough sand rock. If the springs were silica-rich, sarsen is the resulting rock; hard and flinty, perhaps one of the toughest of rock types. BUT, limestone shell debris can be seen within this block and it does react with dilute hydrochloric acid indicating that the cement is not silica but calcite. Perhaps it should be called a calcareous sandstone.