Geological cross-sections
Sedimentary rocks build up layer by layer, like the pages of a book written from the bottom up. Scientists use this layered evidence to build the geologic time scale (NGSS MS-ESS1-4). A geological cross-section is, in that sense, the open book: a small number of simple principles is enough to read the order in which everything happened.
The principles of stratigraphy
If you look at a road cut along a highway in the Appalachians you can see layers of limestone tilted at $40^\circ$, yet if you look today at the bed of Lake Michigan or Chesapeake Bay, the sediments always lie flat: how did those layers come to lose their horizontal arrangement?
State the principle of original horizontality and explain what must have happened if today we find tilted or folded strata.
State the principle of original horizontality and explain what must have happened if today we find tilted or folded strata.
Four undisturbed horizontal strata appear in a cross-section: sandstone, mudstone, limestone, and conglomerate, from bottom to top. Apply the principle of superposition…
Four undisturbed horizontal strata appear in a cross-section: sandstone, mudstone, limestone, and conglomerate, from bottom to top. Apply the principle of superposition to state which is the oldest and which is the youngest, and justify your answer.
Interpreting a geological cross-section in full
In the geological cross-section shown in the diagram, five strata (A, B, C, D and E), an erosion surface (W) and a fault (X) are indicated. Order these events from…
In the geological cross-section shown in the diagram, five strata (A, B, C, D and E), an erosion surface (W) and a fault (X) are indicated. Order these events from oldest to youngest: formation of the five strata, erosion that caused W, and movement of fault X. State which principle you apply at each step.
A fault cuts through a series of strata and, in turn, is cut by an igneous dike. Apply the principle of cross-cutting relationships to order these three events:…
A fault cuts through a series of strata and, in turn, is cut by an igneous dike. Apply the principle of cross-cutting relationships to order these three events: formation of the strata, movement of the fault, and intrusion of the dike.
Unconformities
In the Grand Canyon, perfectly horizontal strata rest on top of tilted layers that have been cut off at an angle (the Great Unconformity): what geological event explains why millions of years of record are missing between the two packages of rock?
The principle of correlation and index fossils
Two cliffs thousands of miles apart show identical sandstones, but one contains ammonite shells and the other trilobite remains: how can we know which was deposited first if there is no physical continuity between them?