G. S. Thomas starts his fine study of rock gardens by considering their forebears, the grottoes of old designed as garden ornaments. One of the problems that grotto designers had to solve was not so much how to construct and decorate the inside of artificial caverns—with coloured stones and pebbles, stalactites, semi-precious minerals, crystals or shells, as did the Italian or French rocailleurs—but how to cope with their outside look and outer superstructures, how to adorn the mounds over them, so as to turn them into pleasant or interesting garden features.2