Principle of superposition: when comparing two sedimentary rock layers, the bottom layer must be older than the layer above it. the logic is a lower layer must already exist for another layer to be deposited on it. principle of original horizontality: sedimentary rocks were originally deposited as flat-lying, horizontal layers. if a layer is tilted, it must have moved. principle of cross-cutting relationships: any rock or feature, cutting through another rock or feature, must be younger than the material through which it cuts.