Gravel, the name given to aggregations of water-worn and rounded fragments of rocks, varying in size from a pea to a hen's egg. When the fragments are smaller, the deposit is sand; when larger, it is called shingle. Beds of gravel occur in formations of every age. While the materials have been a long time in being prepared, and have travelled perhaps a great distance from the mother-rock, gravel deposits have been formed speedily and by the action of a strong current of water. They form very irregular and limited deposits, occurring generally as banks or hummocks in strata of sand. Unless in the most recent deposits, they almost always form a hard rock called conglomerate or puddingstone, the pebbles being compacted together by some infiltrated cement, either calcareous, ferruginous, or siliceous in character. Even recent gravels are sometimes formed into a compact concrete, though these and later deposits are generally loose. The stones of a gravel or conglomerate may be fragments of almost any kind of rock; but the harder species are the most common—pebbles of quartz and quartzite forming as a rule the chief material in gravel-beds of all ages. In our own day gravel and shingle are formed both by fluviatile and marine action, and the same was the case in the older periods of the earth's history. Thus certain conglomerates mark out for us the sites of old sea-coasts, while others represent old river-beds.
Gravel varies much in character and appearance according to the formation from which it is derived. In the making of roads and walks, particularly in gardens, pleasure-grounds, and public parks, it is the last ingredient used. Essential qualities in a good gravel are (1) that it should be binding—that is to say, it should not shift like sand under foot; (2) it should be durable; and (3) its colour should be agreeable to the eye and in harmony with vegetation. It is rare to find a gravel in which all these qualities are combined. The only sort known in Britain to possess them all in itself is the famous Kensington gravel, which has long been regarded by landscape-gardeners at home and on the Continent as the most perfect natural walk or road-finishing material obtainable anywhere. It is a pit-gravel, and abounds in oxide of iron, to which it owes its binding quality and also its fine warm harmonious colour. Many other pit-gravels also possess this cohesive property in a high degree, but are defective in colour. As possessing better binding properties, pit-gravels generally are to be preferred to sea or river gravels; but their defects of colour often preclude their use in landscape-gardening. The Kensington gravel is costly and difficult to procure. On this account, and also because of its similarity in colour, the most popular gravel of the present time is the Dorset Pea; but it is also one of the most shifting, the flinty pebbles composing it being round and about the size of a pea. As the name implies, this sort comes from the coast of Dorsetshire. From the shore of the neighbouring county, Hampshire, is obtained another pleasingly coloured flint-gravel named the Lymington; and the Sussex coast furnishes two sorts named Sussex Pea and Sussex Bean. The prevailing form of the former is pea-like, that of the latter bean-like; hence their respective names in commerce. They are found commingled on the shore, and are separated by sifting. Shell-gravel—so called because composed of minute shells entire or the fragments of larger ones—is also a favourite gravel, being pleasing in colour and comfortable to walk upon when not laid on very deep. It is found on various parts of the British coasts and on those of the Channel Islands. Musselburgh gravels—both shore and pit—are prized in that district, being good in colour, and the pit variety has also fair binding properties. There are many manufactured gravels, such as granite, whinstone, marble, quartz, slag, glass, &c., which are crushed in machines, and afterwards riddled to the desired sizes. These and all the sea and river gravels are used in making asphalt and other composite roads and paths, some of them when skilfully combined with cement imparting a very beautiful appearance to the surface.