California Code, Government Code - GOV § 23155

The boundaries of Tuolumne County are as follows:

Beginning at the most western corner, being the southern corner of Calaveras, in the Stanislaus River;  thence southeasterly to the common corner of Merced, Mariposa, Stanislaus, and Tuolumne;  thence easterly on the northern line of Mariposa and Madera, following the summit line of the dividing ridge between the Tuolumne and Merced Rivers, to Mount Lyell, as marked on Warren Holt's map, 1869, and the summit of the Sierra Nevada Mountains, being on the western line of Mono and the common corner of Tuolumne, Madera, and Mono;  thence northerly by the line of Mono, being the summit line of the Sierra Nevada Mountains, to the southern corner of Alpine;  thence northwesterly by the line of Alpine to the southeastern corner of Calaveras;  thence westerly on the line of Calaveras and down the Stanislaus River to the place of beginning.