Relative dating

Objective: Explain superposition and original horizontality, distinguish their meanings, and apply them to a described case.

Superposition: In an undisturbed sedimentary sequence, lower layers were deposited before layers above them. Original horizontality: Sediments are generally deposited approximately horizontally, so later tilting implies subsequent change. Relative-dating principles require attention to geological context. Superposition applies most directly to sequences whose original order has not been overturned. A cross-cutting feature postdates the material it cuts, but later events can modify both. An unconformity indicates a gap rather than a known duration by itself. Build a sequence from several relationships and state where the record is incomplete instead of inventing missing events. Application: An intact lower sandstone layer predates the overlying shale. This most directly illustrates superposition. Tilted strata indicate deformation after deposition. This most directly illustrates original horizontality.

Objective: Explain cross-cutting relationship and inclusion, distinguish their meanings, and apply them to a described case.

Cross-cutting relationship: A feature cutting another geological feature is younger than the feature it cuts. Inclusion: A rock fragment enclosed within another rock existed before the enclosing rock formed. Relative-dating principles require attention to geological context. Superposition applies most directly to sequences whose original order has not been overturned. A cross-cutting feature postdates the material it cuts, but later events can modify both. An unconformity indicates a gap rather than a known duration by itself. Build a sequence from several relationships and state where the record is incomplete instead of inventing missing events. Application: A fault crossing rock layers formed after those layers. This most directly illustrates cross-cutting relationship. A conglomerate contains older pebbles. This most directly illustrates inclusion.

Objective: Explain unconformity and relative chronology, distinguish their meanings, and apply them to a described case.

Unconformity: An unconformity represents missing time caused by erosion or nondeposition. Relative chronology: Relative chronology orders events without necessarily determining their numerical ages. Relative-dating principles require attention to geological context. Superposition applies most directly to sequences whose original order has not been overturned. A cross-cutting feature postdates the material it cuts, but later events can modify both. An unconformity indicates a gap rather than a known duration by itself. Build a sequence from several relationships and state where the record is incomplete instead of inventing missing events. Application: A surface separates older eroded strata from younger deposits. This most directly illustrates unconformity. A student orders deposition, faulting, and erosion. This most directly illustrates relative chronology.