Most of these features that rise more or less vertically through other rocks are volcanic in origin (see our post on Ship rock at http://on.fb.me/1GZOKfo), consisting of lava that has been squirted upwards through the crust, usually along pre existing faults or lines of weakness. The sedimentary versions are more complex, forming via a variety of processes. They are found all over the world in a wide variety of rock types. They vary from centimetres to metres in width, with the average being a few cm. The most obvious variety forms when a natural crack in the ground formed by erosion (such as frost wedging or dissolution of limestones), or an earthquake, gets filled with a different kind of sediment.