2 4 16 Stratigraphy the study of relationships between rocks 1 Unconformities a break in a rock sequence that represents a period of erosion and unconformity represents a time gap in the rocks deposition stopped time has elapsed but no record of that time possible that some unknown amount of rock eroded often a boundary between groups sequences of rocks Two main types of unconformities in sedimentary rocks A Disconformities Break in flat lying rock sequences Deposition stopped time elapsed B Angular unconformity uplift even tilted the lower sequence before deposition resumed not just a break in deposition and some erosion uplift even tilted before deposition resumed C Special case nonconformities Magma chamber intruding into sedimentary rocks so that the bottom layer of rock the magma chamber The bottom layer is younger than the top layer the sedimentary rocks 2 Crosscutting Relationships dikes faults angular unconformity all are crosscutting 3 The Law of Inclusions Inclusion piece of a rock within another rock The inclusion is always older than the rock it is in The rock containing the inclusion is younger