Sun-cracks

Sun-cracks, superficial markings frequently seen on the surfaces of thin bedded flagstones and argillaceous sandstones. They are believed to have been formed in the same way as the fissures which are produced upon the mud-flats of tidal rivers or estuaries by the drying and shrinking of the deposits during their temporary exposure at low tide. The cracks are of course filled up by new deposits when the mud-flats are again overflowed. Not infrequently the material filling the sun-cracks is of harder consistency than the rock in which they occur. When the bed overlying the cracks is removed a cast of these often projects from its under surface, or frequently the casts remain in the moulds so as to form a series of polygonal ridges ramifying over the whole surface of the exposed stratum.