The Sarsen Witch — Eileen Kernaghan

Naeri, the only survivor of her tribe, lives by her wits. Then she is captured by the horse-lords who have supplanted her people. She seems destined for a life of slavery -- but the Mother watches over her own. Naeri can hear the voices of the stones; her birthgift is geomancy, the earth-magic. It is Naeri who who guides the building of the great stone circle. A Great Chief thinks it will stand eternal testament to his might, but Naeri knows the circle will harness the forces of earth and sky . It can be used to destroy the horse-lords and restore the rule of the witchfolk....

The Sarsen Witch is the third volume in the award-winning Grey Isles trilogy, which also includes Journey to Aprilioth and Songs from the Drowned Lands.