The accompanying figure gives a clear idea of the sequence of beds drawn from a photograph which we owe to the kindness of the Abbé Bonno.
These gravel quarries are excavated in the Pleistocene or Quaternary gravels and are situated on a plain between 125 and 140 feet in height
above the sea, extending from Chelles eastward to the neighboring village of Brou. The strata underlying the fresh water or river-gravels are greenish clays or marls of marine origin and of Eocene Tertiary age. This marl contains gypsum or plaster of Paris, with crystals of