Chalk: nf Chalk, latin creta, is a sedimentary rock of marine origin formed from an accumulation of shells and calcareous micro-organisms in a shallow sea (50 to 100 m), where sea urchins, oysters lived and belemnites (fossil cephalopods close to the squid). The sea then withdrew and this calcareous mud solidified to constitute the chalk and its chalky ground. It gave its name to a geological period, the Cretaceous (60 to 80 million years)...
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