《水经注·卷十八》
此外，旧藐县有杜阳山，山北有杜阳谷，谷中有个地洞，向北通进去，也不知尽头在哪里，地点在天柱山以南。
又故虢县有杜阳山，山北有杜阳谷，有地穴，北入，亦不知所极，在天柱山南。
In addition, the old Miao County has Duyang Mountain, and there is a Duyang Valley in the north of the mountain. There is a cave in the valley toward the north, but it is unknown where it ends, and it is to the south of Tianzhu Mountain.