Gal Potha (Stone Book) is one of the well-known works of King Nissankamalla (1187-1196) which depicts himself, his rule and the eligibility for being a king of Sri Lanka found next to Sathmahal Prasada. This gigantic slab which is 26'10" feet, 8.2 meters in length and 4'7" feet, 1.4 meters in breath has been brought to structure Mahiyanganaya zone by the worriers of King Nissankamalla (1187-1196). The content is written in 3 columns and contain more than 4300 characters in 72 lines. Inscriptions show that the inscription had been raised on a brick podium underpinned with short pillars and sheltered by a canopy supported by ten stone pillars. There are signs that liquidized metal has been poured on to the carved characters to bring out the letters much more clearly.
Excellent carvings of two lines of geese (Hamsa) form the outskirt of the side of the stone. Between this designs at the center, is an engraved figure of a seated Goddess holding two flowers and two elephants pouring water on her from a holder. This figure has been identified as the Goddess of Prosperity, Gajalakshmi.
The inscription also states that the large granite slab was transported from Segiriya, which is now the present Mihintale, by the strong men of King Nissankamalla under the leadership of Adhikara Totadanavu Mand-navan. The scholars have dated it to the ninth year of the reign of King Nissankamalla.