Bands of flint are found embedded in chalk and other kinds of soft limestone.
When the chalk is eroded, the hard flint nodules survive as pebbles on a shingle beach. It may happen that the pebbles later get cemented into another rock, such as a puddingstone. In this way, they make up a sedimentary rock for the second time.
↑Frere, John. 1800. Account of flint weapons discovered at Hoxne in Suffolk. Archeologia13, London. 204-205