Review Geologic "Laws"
Uniformitarianiam
Original Horizontality
Superposition
Other "almost a law" situations exist
Cross-cutting relationships - the "Zucchini Effect"
Weathering and erosion - near surface and above sea level
Results in gaps in the geologic record: Unconformities
Very common in the stratigraphic record
Can be very difficult to recognize
3 types: angular unconformity, disconformity, nonconformity
Put all this together
Videodisc 1934 to 1953
Overhead block diagram samples
Geologists like to figure out the history of the earth
Establishing the relative time sequence in a local area is relatively easy
It's much more difficult on a regional scale
Remember Strickler's 1st Law - it also works in reverse!
Correlation of separated rock units can be tough
Compare rock characteristics - Physical and compositional
Marker beds: wide-spread, regionally distinctive rock units of limited duration
Ash deposits are excellent examples
These greatly aid in developing a regional time scale
Think of what a marker bed man will be!
Unfortunately, not all sedimentary units are 'regionally distinctive'
Fossils are the best and most commonly used correlation tool
Fossils are the preserved remains or traces of prehistoric plants or animals
Rapid evolution, short-lived, wide spread species work best
Very spotty record - Only some relatively young rocks have fossils
Basically, we're talking about the last 600 m.y. or so
Fossils form the basis for the Relative Time Scale
Indicate that life has changed continuously in form and kind
Constantly adapting to changing environments on the earth
Refer to a separate discussion on Paleontology