An estuary is a wide opening at the mouth of a river into which the sea has penetrated by the depression of the land. In such bodies of water the tide often scours with much force. Estuaries abound along our Atlantic coast, Delaware and Chesapeake Bays and the mouth of the Hudson being excellent examples of such. The water in them is brackish, and unfavourable to abundant aquatic life, for only a limited number of marine animals, and fewer fresh-water ones, flourish in brackish water. • Estuarine deposits are, in general, much like those of the sea, except that they are apt to be of a finer grain for a given depth of water; muds are abundantly laid down, especially in the more sheltered nooks and bays, with fine and coarse sands and gravels in the more exposed situations. The sands are apt to show a confused stratification from the conflicting currents and eddies in which they are deposited, but with horizontal layers formed at slack water. Extensive mud-flats often surround an estuary, especially if the rise and fall of the tide be great.

On these flats, exposed during low tide to the sun and air, in dry, hot climates, sun cracks are formed on the drying surface, and these, together with the prints of raindrops and the tracks of land animals, will be preserved when the incoming tide, advancing too gently to scour the slightly hardened surface of the flat, deposits a fresh layer of sediment upon it. In pluvial climates, the mud-flats of estuaries do not dry with sufficient rapidity to permit the formation of shrinkage cracks, except when the flats are exposed to the air for a longer time than the ordinary interval between tides. This longer exposure occurs when part of the flats is covered only by spring tides, or the general level of the water is raised by a storm. If the estuary be the opening of a large river, considerable deposits of river sediment will, in times of flood, be laid down upon the other beds, producing an alternation of fresh and brackish water beds. On the coast of North Carolina the low sand-spits thrown up by the waves enclose extensive shallow sounds, into which the tide enters by only narrow openings, but which have numerous streams flowing into them.

At high water the incoming tide acts as a barrier, damming back the river waters, checking their velocity, and causing them to deposit their burdens of sediment. In course of time, the sounds must be silted up by the rivers, first converted into salt marshes and then into land. The great areas of salt marsh along our Atlantic coast have, for the most part, been formed in this way.

For reasons that we have already discussed, estuaries are not favourable to either fresh-water or marine organisms, and hence estuarine deposits do not contain any great variety of remains of either group. These remains may, however, represent numerous individuals, sufficient sometimes to form limestone layers, as is especially true of oyster banks. Diatoms may also accumulate in great quantities, as in one of the Baltic harbours, where they form 18,000 cubic feet of deposit annually, which necessitates continual dredging to keep the harbour open. On the other hand, estuaries are often favourably situated for the reception and preservation of the remains of land animals and plants which are swept into them by streams and buried in the soft silt of the mud-flats.