Earth-stresses and shrinkage give rise to joints in sandstone, which may not be so clean and sheer as those in limestone, but which affect even the softer forms. Cemented sand-dunes of modern date tend to break away along vertical planes. Firmer sandstones give rise to stepped table-lands and "edges," and the resistance of many types to atmospheric decay renders their stratified structure strongly apparent. Small intervals in the process of deposition, or slight changes in the coarseness of the sand brought down by currents, give rise to laminated and flaggy types. Where a broad shore has been exposed between tide-marks, the drying and compacting of the surface before the next layer is laid down enables the latter to take a mould of the inequalities of that below. Ripple-marks, sun-cracks, rain-prints, and the footmarks of animals, are often preserved in this manner. Where the shore is subsiding, they may persist through hundreds of feet of strata. Naturally, the best examples of these casts, and of the original structure in the underlying bed, occur where a little mud has been laid down over the sandy flat. Clay by itself, if damp, does not retain the impressions sufficiently long, and, when once thoroughly dried, it crumbles when the next water overflows it. But a foundation of firm sand with a thin mud-layer on its surface, as may be recognised in some Triassic deposits, furnishes excellent records of local weather or of the movements of errant animals. On the flat shores of lakes in a semi-arid climate, the water may retreat for miles, and return, perhaps months afterwards, when rains in the hills have given it a new burden of detritus. Under such conditions, broad sun-cracked flats may be preserved, with perhaps some plant-remains between successive layers[938]. The castings and tracks of worms, and the tubes of boring species, which are sometimes infilled by sand of a different colour, are common in sandstones of all ages. SILICEOUS CONGLOMERATES