Revised 8 / 06 (Monroe 6th ed.)
Including...
Introduction to Paleontology
Evolution and natural selection
Evolution and Geologic Time
Relative Age Dating
Stratigraphic Columns
Stratigraphic Correlation
Study of earth history based on fossils
A study of fossil remains indicates that life forms on earth "have evolved from relatively simple to complex hierarchies of plants and animals"
Used by Darwin (and others) in the formation of the theory of evolution
Fossils have proven to be exciting throughout recorded history (and pre-history)
Their true meaning, however, hasn't always been recognized
Avicenna (980-1037) - a disciple of Aristotle: they grow within the rocks and resemble bones and plants only by chance
Dogma of the middle ages: They were created by Satan to confuse us
William Smith (1769-1839) - English canal builder
Spent 24 years mapping canal routes and observing the rocks
First established the relationship between stratified rocks and the fossils they contain
He published the first geological map in 1815
Failed to recognize the evolutionary significance of the fossils
To him they were essentially just distinctive shapes which allowed him to assign relative orders to the rock units in the area
Fossilization
Several different processes:
Unaltered hard (or soft) parts - pretty rare
Preservation: Rancho la Brea Tar Pits
Amber - like in Jurassic Park
Desiccation - mummies
Frozen - mammoths
Altered hard parts - Your basic fossil
Replacement: usually by calcium or silica
Molds and casts: filled with silt
Trace fossils: tracks, burrows, etc.
Coprolites: fossilized dung
Classification: obviously different creatures so they need different names
Same basic idea as system used in biology
Kingdom, Phylum, Class, Order, Family, Genus, Species, Individual
Used as the basis for the divisions of the relative time scale
Therefore pretty important to geologists
Unfortunately, very spotty record with lots of holes!!
Lots of different species have lived and died on earth throughout geologic history
We're not going to go into great detail, but a few obvious examples include:
Dinosaurs (w/ Jurassic Park film clip?)
Ammonites (key to videodisc)
Trilobites (w/ samples)
Banded Iron Formation (see separate discussion)
As mentioned earlier...
A study of fossil remains indicates that life forms on earth "have evolved from relatively simple to complex hierarchies of plants and animals"
Basic Darwinian theory: explain
Charles Darwin (1809-82)
Geologist/Biologist on the HMS Beagle in 1859
Quite a guy!
Wrote "The Origin of Species" based on his observations on that voyage
This has resulted in many similarities between different species
Not unexpected - some things just work well and are in common usage
Such as: 2 eyes, 2 ears so we can calculate distance
Also, similar body parts doing similar things
Called "homologous structures"
Life appears to have evolved from simple to more complex forms
Logical progression?
Changing environmental conditions
Addition of free oxygen in the water and atmosphere
Expansion of life into broader niches
Ammonite sutures and water depth
Nature continues to adapt life to the constantly changing surface environment
Has resulted in some amazing patterns in evolution
DIGRESS TO: our BRIEF and LIMITED glimpse of the past!
Not much in the way of "Laws" in geology
To review: the classics include...
Uniformitarianism - 'the present is the key to the past'
If a process is active today, it's probably worked the same throughout geologic time
Gravity, water running downhill, that kind of thing
Principle of Superposition
"in any sequence of layered rocks any one layer will be older than the layer above it and younger than the layer below it"
This can be used to determine the relative ages of undisturbed sedimentary sequences
The key term here is "undisturbed sedimentary sequences"
Faulting and folding can play merry hell with superposition
Principle of Original Horizontality
States that sedimentary beds are laid down horizontally (or near horizontally)
If they aren't flat, some tectonic force must have disturbed them
Define tectonics
There are definitely exceptions to this "law"
Principle of Cross-cutting relationships - the zucchini concept
"if one body of rock cuts across another body of rock, the latter must be older than the former."
In simple terms, nothing can be done to an object which does not exist
Therefore, the thing being acted upon has to be older than the thing doing the acting
Miscellaneous Principles:
Principle of Lateral Continuity
Principle of Inclusions
Principle of Faunal Succession
Weathering and erosion of pre-existing rock or sediments
Can only happen near the surface of the earth
This "law" is really a negative situation
Based on the absence of any rock record
Results in a gap in the geologic record
Unconformity
"results from a period of erosion in the geological past during which previously formed rocks were lost, followed by a period of deposition of additional layers of rocks"
Therefore, always indicate uplift, erosion, and subsidence
These are very common in the stratigraphic record
Can be very difficult to recognize
Especially if there hasn't been any uplift and tilting of the sedimentary layers
Click here for a complete discussion from the Ask GeoMan section
Geologists use relative dating techniques to sort out geologic events
DIGRESS TO: stratigraphy, lithology
Prepare localized graphic representations of the stratigraphy
Called stratigraphic columns
Generally follow the rule of superposition
Oldest unit on the bottom
DIGRESS TO : Formations, units, and related subdivisions
There are some accepted symbols used to designate various lithologies and features
DIGRESS TO: stratigraphic symbols
Lots of information contained on a GOOD stratigraphic column
Lithologies
Age relationships
Erosional characteristics
Unconformities
Geologists use these to put together regional correlation of geologic events
And therefore the geologic history of an area
Much more on this later
DIGRESS TO: Marker beds
Establishing the relative time sequence in a local area is relatively easy
Any given outcrop is usually pretty straight-forward
It's much more difficult on a regional scale
DIGRESS TO : regional vs. local
The best method to be used in correlating separated rock units is to compare the characteristics of the rocks themselves
Physical and compositional
The more distinctive, the better
Marker horizons
These are regionally distinctive rock units, fossils, or mineral assemblages
To be a good marker horizon:
Regional in extent
Cover a wide area
Regionally distinctive
Retains its unique characteristics wherever found
Represent a relatively short time
A species which stays the same for 123 million years isn't much help
Ash deposits are excellent examples
Often satisfy all 3 requirements
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'
Many, if not most, change laterally into subtle to profoundly different types of rocks
Pebble Conglomerate to Sandstone to Shale to Limestone
Near shore high energy zone to quiet, deep water carbonate sedimentation
Facies Changes: much more on these later
Fossils are the best and most commonly used correlation tool
Rapid evolution of short-lived species works best
Unfortunately, only Phanerozoic rocks have fossils
And not even all of those have fossils
Basically, we're talking about the last 600 m.y. or so
Fossils allow geologists (Paleontologists) to correlate similar rock units across very wide distances
They form the basis for the Relative Time Scale
Fossils show clearly that life has changed continuously in form and kind throughout geologic time
Constantly adapting to changing environments due to changes in the earth
GeoMan's Home Page | RCC Index | High School Geology Index
You are GeoManiac number since April 1, 1997