Depending on one’s allegiances, Terry Riley’s In C, written in 1964 for an indeterminate number of musicians playing unspecified instruments for an indeterminate length of time, is either a cornerstone of American minimalism or the cornerstone. A few months shy of his 85th birthday, the composer and still globe-circling pianist celebrates with his son Gyan Riley, guitar, and his longtime champions, the Kronos Quartet, in the heart of Gold Rush country in the foothills of his native Sierra Nevada. —M.G.