Ahtanum Ridge is a long anticline in Yakima, Washington, lying just south of the city and along much of the northern edge of the Yakama Indian Reservation. Its name derives from the Sahaptin place-name átanɨm. The ridge is part of the Yakima Fold Belt, a series of long east-trending ridges formed when Miocene Columbia River basalt flows were folded. It divides the Yakima Valley to the south from the Ahtanum Valley to the north. At its eastern end, the Yakima River passes through the ridge at Union Gap, and the landform continues eastward beyond the gap as the Rattlesnake Hills.