The Externsteine are located on the northeastern slope of the Teutoburger Wald. They are a natural outcropping of sandstone in a region that is otherwise largely devoid of rocks. The formation stretches for several hundred metres. It starts inside the forest with some rocks that are mostly covered by soil. It terminates in a series of 13 highly visible and mostly free-standing pillars. For scientific purposes, these have been numbered I-XIII from northwest to southeast with the most imposing being stones I-V. The largest is rock I (subdivided into Ia and Ib), also known as the Grottenfels due to its cave. The next one, II, is also known as Turmfels (tower) and rises above the surrounding area. Rock III is called Treppenfels (stairs). A large gap separates rocks III and IV. The latter is also known as Wackelsteinfelsen due to a loose boulder (now fixed in place) at its top.: