Around 17 million years ago, when prehistoric rhinoceros and camel roamed this region, a chain of geologic events began that drastically changed the landscape. In a warmer climate before the eruptions, forests of ginkgo, elm, and cypress grew here. Then long cracks called fissures opened in the earth's crust and repeatedly spewed spreading pools of basaltic lava across the land, destroying everything in their path. Over millions of years, these eruptions built layer upon layer of basalt, in some places almost a mile thick. As the lava gradually cooled, it cracked into vertical columns, with slow, even cooling producing straight, wide columns and rapid cooling creating a more jumbled appearance. Later, erosion stripped away topsoil and exposed cliffs, leaving Hat Rock and similar distant outcrops as visible parts of the same ancient flow.