Two examples of natural bridges cut in basalt lavas are described. Upstream of both examples is a gorge and a dry waterfall. Strearnwater bypasses the waterfall via enlarged joints in the flow. These features seem to have been initiated by collapse and extension of lava tube systems and the arches represent stronger sections of tube roof that have survived. Erosion and enlargement of joint planes has been important in developing these features and presently this may be the major erosional process in these valleys.