Understanding Fossils
Part 1...
First you may ask, what is a fossil?
Fossils are the remains of prehistoric life or some other direct evidence that such life existed.
How are fossils formed?
Fossils are formed in a variety of ways. In some instances, the whole organism has been preserved, as in the case of the well-known frozen mammoths of siberia, whose flesh, when found, was still edible. But most fossils are only the partial remains of animals or plants- generally the hard parts of the organism that were buried quickly and thus preserved from decay.
Fossils are divided into these catagories-
Molds:
Given enough time even the hard parts of a fossil may dissolve leaving only the cavity. These are walls of which are a natural mold of the fossil. Such casts are common fossil forms.
Steinkerns:
If the interior of a shell fills with mud that hardens and becomes free of the surrounding matrix, the resulting internal mold is called a steinkern. Since a steinkern is formed in the interior of a shell, it does not show the outer form or surface features.
Casts:
If the cavity left by the whole shell has filled in with a mineral such as calcite, silica, or pyrite, the result is called a cast.
Petrifications:
Often the original material is replaced by minerals, which gradually seep into the fossil from the surrounding matrix. When this happens, the resulting fossil is called a petrification. Wood and bone are often preserved this way.
Films:
Leaves are frequently preserved as carbon films on rock surfaces, left after the other substances that made up the original leaf have been dissolved away. Only the more stable carbon remains, usually as a black or brown film that records the exact shape and venation of the leaf. Occassionally animal fossils are also preserved as films, but the material that remains may not be carbon, even when it is black.
Usually a leaf or animal preserved by distillation leaves an impression in the rock as well. The carbon film may be washed away before it is collected, so that all that remains is this impression.
Trace:
Sometimes a fossil is not part of the animal itself, but a trace of the animal's activities.
These include-
Borings of worms and mollusks.
Gastroliths which are found inside or near the rib cages of dinosaurs. These stones probably aided the dinosaur digestion just as gravel in their gizzard helps a chicken crush grain. Polished gastroliths are found only in "dinosaur country".
Corprolites which are fossil excreta and gives clues to the diet of ancient animals.
Foot Prints show animal locomotion and help trace the ecologies of the ancient world.
Artifacts are stone tools and weapons made by ancient man.
Other traces fossils include indications of feeding activity, or the burrows once occupied by prehistoric animals.
Can fossils form in lava?
Yes! Although very rare, these leave impressions similar to molds or casts. It is more common and easier for an animal to fossilize in volcanic ash than in molten magma.
What else can be a fossil?
Although not a true fossil, Lightning bolts have been known to leave trace remains inside the earth. I will post more on this when I get a chance!
What is a pseudofossil?
Pseudofossils are rock structures that resemble fossils. They may have any shape and often look like parts of plants or animals. A geologist will usually recognize a pseudofossil at once, but an amateur may be misled.
Pseudofossils resemble fossils only in external form. They never have the detailed structure of true fossils. Pseudofossils are formed in may ways. Some are water-worn fragments of rock. Concretions which form in sedimentary rock may contain a fossil, though most do not. Concretions, harder than the rock in which they occur, are often found on the surface. Some minerals form dendrites or fernlike deposits on or in rocks. Moss agates are dendrites, not fossil moss.
What is a "polystrate fossil"?
The word polystrate is defined in the Creationist literature.
"...polystrate trees (trees extending through two or more strata, each of which, according to evolutionary interpretations, was deposited slowly over a long period of time)."
--Duane Gish, Creation Research Society Quarterly 12(1):34-46 June, 1975
These fossils are reasonably common, and have been mentioned in the scientific literature for well over a century, under the name upright fossils or in situ trees. No well-read geologist finds them surprising, and no geologist has ever claimed that it took millions (or even thousands) of years to bury them. The strata associated with polystrate fossils invariably show evidence of relatively rapid deposition. Science is perfectly happy with the idea that deposition is occasionally rapid.
Polystrate trees show every sign of extremely rapid burial, generally when rivers flood over their banks. This is the result of natural processes accompanying river course change.
Big trees commonly stay upright for years and even decades after death. Some "drowned" trees can even keep living, if they are tolerant of waterlogged conditions.
Some upright fossils had rotted-away interiors by the time their burial was final. So, in those cases, the tree had stood dead for some time. The typical height of upright fossils is on the order of two meters, so many of these fossils represent only the base of the original tree. The top of the tree presumably rotted.
How do you date fossils?
*Dating fossils can be done in multiple ways.
Superposition of sedimentary rocks indicates their releative ages. In undisturbed sections, younger rocks overlie older.
Stratigraphic Correlation of strata in one place with those of the same age, deposited at the same period of time in another place, is fundamental in the interpritation of geologic history.
Fossils themselves are important in correlation of sedimentary rocks. The absolute age of rocks is accurately determined by the use of radioisotope dating. It is true that the relative ages of strata were determined prior to the development of radiometric dating, but radiometric dating has confirmed these relative dates to be correct. (i.e. index fossils)
Rock Facies , the sum total of the characteristics of a rock's despositional envirionment, are independant of geological time. An awarness of them, however, is important in correlation.
(1)Lithological Correlation uses the similarity of minerology, sorting, structure, bedding, sequence, and other similar features as indications of similar ages of rocks. Limited use though...
Standard Geologic Column which has been built up by combining rock sequences from different areas, can be matched with a time scale based on measured absolute ages of rocks.
(1)Geophysical Correlation makes use of similarity of physical rock properties as an indication of similar age. limited use though...
Rock Systems in the geologic column are major divisions of rocks deposited during a particular period of geologic time.
(1)Lithographical Correlation is of limited value, since rocks of different lithology often are deposited at the same time in an adjacent area. Geophysical Correlation is limited by the same factors of Lithographical Correlation.
*The accuracy of the empirical basis of the "geologic column" is daily re-confirmed by its use in the petroleum industry to actually produce tangible results. Since the existence of a given layer in the geologic column depends on a location being submerged during the corresponding time period, and the layer not subsequently being eroded away, it is not surprising that any given location would have only a limited number of layers. Thus, the the column as a whole is constructed from data from many locations. Nevertheless, there are locations which contain layers representative of all major periods in the last 500 million years. Furthermore, radiodating confirms the validity of the column.
This concludes Part 1. Each thread window has a set number of words it can hold. I will post more later...
Questions anyone?
Part 1...
First you may ask, what is a fossil?
Fossils are the remains of prehistoric life or some other direct evidence that such life existed.
How are fossils formed?
Fossils are formed in a variety of ways. In some instances, the whole organism has been preserved, as in the case of the well-known frozen mammoths of siberia, whose flesh, when found, was still edible. But most fossils are only the partial remains of animals or plants- generally the hard parts of the organism that were buried quickly and thus preserved from decay.
Fossils are divided into these catagories-
Molds:
Given enough time even the hard parts of a fossil may dissolve leaving only the cavity. These are walls of which are a natural mold of the fossil. Such casts are common fossil forms.
Steinkerns:
If the interior of a shell fills with mud that hardens and becomes free of the surrounding matrix, the resulting internal mold is called a steinkern. Since a steinkern is formed in the interior of a shell, it does not show the outer form or surface features.
Casts:
If the cavity left by the whole shell has filled in with a mineral such as calcite, silica, or pyrite, the result is called a cast.
Petrifications:
Often the original material is replaced by minerals, which gradually seep into the fossil from the surrounding matrix. When this happens, the resulting fossil is called a petrification. Wood and bone are often preserved this way.
Films:
Leaves are frequently preserved as carbon films on rock surfaces, left after the other substances that made up the original leaf have been dissolved away. Only the more stable carbon remains, usually as a black or brown film that records the exact shape and venation of the leaf. Occassionally animal fossils are also preserved as films, but the material that remains may not be carbon, even when it is black.
Usually a leaf or animal preserved by distillation leaves an impression in the rock as well. The carbon film may be washed away before it is collected, so that all that remains is this impression.
Trace:
Sometimes a fossil is not part of the animal itself, but a trace of the animal's activities.
These include-
Borings of worms and mollusks.
Gastroliths which are found inside or near the rib cages of dinosaurs. These stones probably aided the dinosaur digestion just as gravel in their gizzard helps a chicken crush grain. Polished gastroliths are found only in "dinosaur country".
Corprolites which are fossil excreta and gives clues to the diet of ancient animals.
Foot Prints show animal locomotion and help trace the ecologies of the ancient world.
Artifacts are stone tools and weapons made by ancient man.
Other traces fossils include indications of feeding activity, or the burrows once occupied by prehistoric animals.
Can fossils form in lava?
Yes! Although very rare, these leave impressions similar to molds or casts. It is more common and easier for an animal to fossilize in volcanic ash than in molten magma.
What else can be a fossil?
Although not a true fossil, Lightning bolts have been known to leave trace remains inside the earth. I will post more on this when I get a chance!
What is a pseudofossil?
Pseudofossils are rock structures that resemble fossils. They may have any shape and often look like parts of plants or animals. A geologist will usually recognize a pseudofossil at once, but an amateur may be misled.
Pseudofossils resemble fossils only in external form. They never have the detailed structure of true fossils. Pseudofossils are formed in may ways. Some are water-worn fragments of rock. Concretions which form in sedimentary rock may contain a fossil, though most do not. Concretions, harder than the rock in which they occur, are often found on the surface. Some minerals form dendrites or fernlike deposits on or in rocks. Moss agates are dendrites, not fossil moss.
What is a "polystrate fossil"?
The word polystrate is defined in the Creationist literature.
"...polystrate trees (trees extending through two or more strata, each of which, according to evolutionary interpretations, was deposited slowly over a long period of time)."
--Duane Gish, Creation Research Society Quarterly 12(1):34-46 June, 1975
These fossils are reasonably common, and have been mentioned in the scientific literature for well over a century, under the name upright fossils or in situ trees. No well-read geologist finds them surprising, and no geologist has ever claimed that it took millions (or even thousands) of years to bury them. The strata associated with polystrate fossils invariably show evidence of relatively rapid deposition. Science is perfectly happy with the idea that deposition is occasionally rapid.
Polystrate trees show every sign of extremely rapid burial, generally when rivers flood over their banks. This is the result of natural processes accompanying river course change.
Big trees commonly stay upright for years and even decades after death. Some "drowned" trees can even keep living, if they are tolerant of waterlogged conditions.
Some upright fossils had rotted-away interiors by the time their burial was final. So, in those cases, the tree had stood dead for some time. The typical height of upright fossils is on the order of two meters, so many of these fossils represent only the base of the original tree. The top of the tree presumably rotted.
How do you date fossils?
*Dating fossils can be done in multiple ways.
Superposition of sedimentary rocks indicates their releative ages. In undisturbed sections, younger rocks overlie older.
Stratigraphic Correlation of strata in one place with those of the same age, deposited at the same period of time in another place, is fundamental in the interpritation of geologic history.
Fossils themselves are important in correlation of sedimentary rocks. The absolute age of rocks is accurately determined by the use of radioisotope dating. It is true that the relative ages of strata were determined prior to the development of radiometric dating, but radiometric dating has confirmed these relative dates to be correct. (i.e. index fossils)
Rock Facies , the sum total of the characteristics of a rock's despositional envirionment, are independant of geological time. An awarness of them, however, is important in correlation.
(1)Lithological Correlation uses the similarity of minerology, sorting, structure, bedding, sequence, and other similar features as indications of similar ages of rocks. Limited use though...
Standard Geologic Column which has been built up by combining rock sequences from different areas, can be matched with a time scale based on measured absolute ages of rocks.
(1)Geophysical Correlation makes use of similarity of physical rock properties as an indication of similar age. limited use though...
Rock Systems in the geologic column are major divisions of rocks deposited during a particular period of geologic time.
(1)Lithographical Correlation is of limited value, since rocks of different lithology often are deposited at the same time in an adjacent area. Geophysical Correlation is limited by the same factors of Lithographical Correlation.
*The accuracy of the empirical basis of the "geologic column" is daily re-confirmed by its use in the petroleum industry to actually produce tangible results. Since the existence of a given layer in the geologic column depends on a location being submerged during the corresponding time period, and the layer not subsequently being eroded away, it is not surprising that any given location would have only a limited number of layers. Thus, the the column as a whole is constructed from data from many locations. Nevertheless, there are locations which contain layers representative of all major periods in the last 500 million years. Furthermore, radiodating confirms the validity of the column.
This concludes Part 1. Each thread window has a set number of words it can hold. I will post more later...
Questions anyone?
