English: Taken June 26, 1997 while visiting Yellowstone National Park for Penn State's geology field camp course. These photos show interesting jointing structures caused by varying rates of cooling within relatively young basalt flows within the park. The jointing creates elongated columns of rock that are polygonal in horizontal section. These are called collonades, and they result from the slowest rate of cooling because they occur at the bottom of the basalt flow. Above the collonades are smaller yet similar (in shape) structures called entablature. The entablature structures disappear at the top of the flow because the top cools rapidly. These basalt flows have undergone hydrothermal alteration which has changed their color to yellow. Hence the name of the park. This photo shows the very bottom of the flow, just below the collonades, where the crushed and baked remnants of vegetation are visible. The structures here are often called "clinkers".