Teaching relative age dating
Explanations: A – folded rock strata cut by a thrust fault; B – large intrusion (cutting through A); C – erosional angular unconformity (cutting off A & B) on which rock strata were deposited; D – volcanic dyke (cutting through A, B & C); E – even younger rock strata (overlying C & D); F – normal fault (cutting through A, B, C & E).
The principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut.
While digging the Somerset Coal Canal in southwest England, he found that fossils were always in the same order in the rock layers.
As he continued his job as a surveyor, he found the same patterns across England.
Photo from Glen Canyon National Recreation Area, Utah.A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "the present is the key to the past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now." The principle of intrusive relationships concerns crosscutting intrusions.In geology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock.The principle of inclusions and components states that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them.For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer.
Geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events.