The principle of superposition in geology states that rock forms horizontally, with the oldest layers of rock lying at the bottom of a formation and newer rock resting on top. For instance, the Kaibab Limestone at the edge of the Grand Canyon is younger than the Toroweap Formation below it.
The idea that rock forms horizontally is at the heart of the principle of superposition formulated by Danish scientist Nicholas Steno in the 1600s. Part of the principle explains that newer rock conforms to the shape of already hardened older rock or fossils. For instance, when new rock forms on top of established rock, it takes on the shape and layers of that older rock.
However, rock layers are not always deposited horizontally if there are slopes involved. Additionally, rock formation in caves can eliminate some of the lower layers. If this cave collapses, the upper layers are suddenly level with the older layers, making it difficult to distinguish the different strata. Other complications occur as a result of tilting, faulting and folding, which is when the layers of rock are distorted for some reason. For instance, molten rock can creep into the cracks between layers and warp the horizontal strata.