Chambers in Prague's sandstones
Sandstones of Prague are remains of a Secondary era sea and belong to a widespread Bohemian creataceous basin. They usually sedimented close to that time's shores, either at the sea itself, or at shallow swamps, which gave origin to sporadic layers of coal. The sandstones are not quite usable for construction, because they do erode quickly. They were used as foundry, plaster and dishwashing sands or to cover floors of some houses, because dirt was then trampled to sand and the sand swept out then. We know descripitons of 19th century's sand-dealers, who travelled around with carts dragged by dogs and sold sand to housewives of Prague. Underground, often wild, mining of sand is characteristic for some parts of Prague like Strahov ("Guardhill"), Hloubětín ("Burrowing"), Střížkov ("Clipston") and Prosek ("Hewt"). Wide labyrinths of narrow passages with many chambers, adits and levels following edge of sandstone plateaus have arisen this way. Some were also used as cellars or hideouts, but it was not their original meaning and they did not connect into long underground roads as local legends say.