Top of the basalt columns showing the classical hexagonal shapes, f...

These glacial striations are carved in the top of a columnar-jointed basalt lava flow. The lava flow was erupted between 200,000 and 100,000 years ago in the Middle Fork of the San Joaquin River valley. The lava flow was then eroded during subsequent glacial periods, as shown by the finely polished and deeply striated surface.