Idealized sketch of a bracketed intrusive. The igneous (molten) rock must be younger than the sedimentary rock (A) it intrudes, and older than the rock (B) that overlies it. The relative age of the sedimentary beds is known from their fossils.
Facsimile reprint of the famous time scale proposed by Arthur Holmes in 1959.
| Time-scale in millions of years | ||
|---|---|---|
| PERIODS | Since beginning of period | Duration of period |
| PLEISTOCENE | ca 1 | |
| .... | ca 1 | .... |
| PLIOCENE | 10 | |
| .... | 11 | .... |
| MIOCENE | 14 | |
| .... | 25 | .... |
| OLIGOCENE | 15 | |
| .... | 40 | .... |
| EOCENE | 20 | |
| .... | 60 | .... |
| PALEOCENE | 10 | |
| .... | 70 ± 2 | .... |
| Upper } | ||
| CRETACEOUS | 65 | |
| Lower } | ||
| .... | 135 ± 5 | .... |
| Upper } | ||
| JURASSIC | 45 | |
| Mid. & Lower } | ||
| .... | 180± 5 | .... |
| TRIASSIC | 45 | |
| .... | 225 ± 5 | .... |
| PERMIAN | 45 | |
| .... | 270 ± 5 | .... |
| Upper } | ||
| CARBONIFEROUS | 80 | |
| Lower } | ||
| .... | 350 ± 10 | .... |
| Upper } | ||
| DEVONIAN | 50 | |
| Lower } | ||
| .... | 400 ± 10 | .... |
| SILURIAN | 40 | |
| .... | 440 ± 10 | .... |
| ORDOVICIAN | 60 | |
| .... | 500 ± 15 | .... |
| CAMBRIAN | 100 | |
| .... | 600 ± 20 | .... |
Using measurements made in many laboratories, and interpolating between them by using the relative thicknesses of sediments, the great British geologist, Arthur Holmes, established the time scale that is in general use today. His original scale is shown on the preceding page. Small changes can be expected to be made in this scale from time to time, but major alterations are not likely, except perhaps in the Cambrian Epoch where the present data are unreliable because they are not complete. (A scale showing the epochs or periods as often given now is on [page 4].)
Precambrian Stratigraphy
So far we have talked only about rocks that are of the Cambrian Epoch or younger—rocks that may contain fossils. Yet there are vast areas (most of Canada, for example) that are covered with rocks older than the Cambrian formation. Some Precambrian fossils have been found, but they are so rare that they are useless for dating the strata containing them. Long-range correlation of Precambrian rocks must rely on nuclear measurements. Therefore it has been only in the last dozen or so years that some order could be established for the Precambrian rock sequences. The elaborate Precambrian stratigraphies (arrangements of strata in sequence) proposed in the past, most of them based on superficial similarities of the rocks in one place to those in another place, now have been drastically altered and in some cases completely overturned by nuclear measurements. We are still far from understanding the sequence of all the events in that vast span of time we call the Precambrian. Many thousands of nuclear age determinations will have to be made to lighten the dark corners of our ignorance.
Folded strata of Precambrian rocks, including limestones and shales, in Glacier National Park, Montana.