Multnomah Falls and Columnar Jointing, I posted this picture taken of Sam at Devil's Tower a few years ago. It remains one of my favorite pictures of geology (and Sam) as it provides such a great image to show several geologic principles at ones. Aside from the geologic signficance of the tower itself, the promionant geologic feature is the massive hexagonal piece that 7-year old Sam (for scale) is leaning against. Behind it, you can see the joints extending up the tower with clear hexagonal blocking roofs, allowing one to recognize the block as having weathered and fallen off the tower.
The picture also has trees and shrubs growing in the fractures below the main part of the tower. These fractures are not the columnar jointing of above, and so demonstrate a different condition of cooling for the base of the tower. The fractures below also have trees and shrubs growing amids the fractures illustrating yet another form of mechanical weathering.