What is "rock sand"

Rock Sand (1900–1914) was a British Thoroughbred race horse and sire. In a career which lasted from the spring of 1902 until October 1904 he ran twenty times and won sixteen races. After being a leading British two-year-old of his generation he became the tenth winner of the Triple Crown in 1903, winning the 2,000 Guineas Stakes the Epsom Derby and the St. Leger Stakes. He won another series of major races as a four-year-old before being retired to stud, where he had success in both Europe and North America.