Rock Types for Rock Gardens
Types of Rock Used in Rock Gardens
The type of rock selected is not at all important, but if nicely weathered natural rock is available, so much the better. Generally speaking, the local rock, if any, fits in with the surroundings more easily than rock imported from farther away, and the nearer the source the lower the cost of transport. Sandstone is very sympathetic to ferns, but it varies a good deal in hardness and configuration, and colour. The Yorkshire sandstone is hard and angular, but can be had from biscuit shades to deep brownish red with purplish shadings. It splits on flat beds, and has vertical cracks almost at right-angles to the base.
At the other extreme, Sussex sandstone is soft, so soft that some pieces can be rubbed into sand in the hands. It should have been laid in the open at least one winter before being chosen, as some pieces break up with the frost; other pieces harden and withstand frost for years. It weathers to a rather dark brown.
Cheshire and some Scottish sandstones are a dull red in colour, which looks well with ferns, but does not always tone in well with some rock-plant colours.
The mountain limestones usually are regarded as the best rocks for effective landscape rock gardens, and there is no doubt that well-selected stone can make a magnificent job; and the silvery-grey colour is beautiful when the stone is fresh from the fells, but away from their own fells transport costs may be high.
But one disadvantage of limestone in a town is that the beautiful weathered colour rapidly disappears in the acid atmosphere, becoming a dead white, with the crevices filled with sooty deposits. Then limestone selected because it is mossy loses the moss in towns and becomes white even more rapidly.
All the above rocks have natural lines of cleavage more or less parallel to the base, and should be placed with their lines ofdipping at the same angle throughout the design. Nothing looks worse to the expert eye than rocks placed with their stratification lines dipping all ways. It may be natural, in that loose rocks may be found any way up, but so are earthquakes, and the object of making a garden is to create a restful picture and not a cataclysm of nature.
Rocks having no definite lines of stratification may be placed that way up which seems most effective, usually with the largest side down, but not always so. Many pieces of mountain limestone come out with a top broader than the base, which may even run to a point.
Tufa is an excellent rock when it can be obtained near at hand, but otherwise it is now one of the most expensive rocks obtainable in Britain. Its shapes seem to join up well, it is sympathetic to plant growth, and it can be placed any way up without looking wrong.
Granites too are not sedimentary; they are sometimes given to fracturing in awkward planes, but they can be built together to make impressive features with a little practice