GEOL 102 Historical Geology

Spring Semester 2018
Physical Stratigraphy

The Great Unconformity at the Grand Canyon: the Cambrian Tonto Group overlaying the tilted beds of the Proterozoic Grand Canyon Supergroup

Many attempts at calculating age of the Earth:

Physical Stratigraphy
Sedimentary rocks, because they are deposited, naturally form horizontal layers (strata, singular stratum). Because of their layered form, strata allow geologists to determine relative time (that is, sequence of deposition of each layer, and thus the relative age of the fossils in each layer). These form the basic Principles of Stratigraphy. he first three principles were developed by Niels Stensen (better known as Nicholas Steno):

Richard Alley (PSU) gives some advice concerning geopetal indicators:

As Steno and others mapped out strata, they found that sometimes there were types of breaks (discontinuities) in the layers. These are called unconformities, and represent gaps in the rock record (periods of erosion and/or non-deposition). Hutton, of Uniformitarianism fame, studied these and recognized that they represented aspects of relative time.

Three main kinds of unconformities:

From unconformities, Hutton added additional Principles of Stratigraphy:

(Here is a great blogpost about Siccar Point and Hutton's discoveries.)

Using these principles, early geologists were able to figure out the sequence of events of deposition, the changing local environments, and the folding, faulting, igneous intrusions, etc. for any particular section of rock. However, how could they extrapolate the sequence at one section with the sequence at another location, far away?

To Lecture Notes.

Last modified: 2 February 2018

Folded Paleozoic strata of the Shell Creek Monolcine, Wyoming