Science
in Christian Perspective
FOSSIL
SEQUENCE IN CLEARLY SUPERIMPOSED ROCK STRATA
Cordelia Erdman
Instructor in Geology
Wheaton College
From: JASA, 2,
(June1950):
13-17.
The Grand Canyon of the Colorado River has often been likened to a book whose
pages reveal the story of many eons of earth history. The story is told not alone
by the relationship of the rocks to one another and the relationship of the river to
the overall structure, but in large measure by the remnants of life preserved in
these rocks. Here on a vast scale and in an accessible form is a completely unprejudiced account of ancient life.
Since paleontology, the study of ancient life, is so frankly geared to
evolutionary philosophy, it sometimes faces the accusation of tailoring facts to fit the philosophy. The present paper is neither intended to be such an accusation nor designed
to be a defense of paleontological method; rather, it is merely an account of a
succession of life forms as they actually occur in the Grand Canyon. This account
will be proffered without aid of any supporting philosophy.
Some simple principles of geology will provide a background for this description:
The most common rocks at the earth's surface are those which are termed "sedimentary." Before becoming rock they are merely loose sediment which has been
deposited in essentially horizontal layers by water or wind, in the sea or on land.
Each layer of sediment, in fact each particle, is deposited in response to certain
very precise chemical and physical conditions, and so long as those conditions
prevail, either locally or over a large area, the resulting sedimentary deposits will
be of one characteristic type and texture. Any change in the controlling conditions
is manifested in a corresponding type of sedimentation which gives the ultimate rock
its own differentiating character or lithology. Such changes may take place abruptly
or gradually. Thus a relatively homogeneous layer of rock represents the persistence
of relatively homogeneous environmental conditions during a short or long period of
time* Vertical successions of such layers are the "strata" of geology,
Marine deposition of sediment, the most common type, cannot take place indefinitely upward. 2ach site of sedimentation has its own governing "base level" an
horizon above which there can be only erosion and below which there can be only
deposition. Thus the presence of a thick series of rock layers shows that the region
must have been sinking in order to make continuous deposition possible.
The sea may gradually withdraw from an area and thereby establish a new and
lower base level. The even surface of the newly exposed layer will then be subject
to attack from forces of erosion, and if this region once more sinks beneath the sea
and sedimentation is renewed, there will be a highly irregular contact between the
old and new layers. Pebbles and boulders of the old lower stratum may be incorporated in the base of the now one,
thus clearly showing the relative ages. The record
of all these events will be preserved in the rock which is formed by a process of
gradual hardening due to pressure from overlying material and the action of cementing
agents. Thus in a vertical senuence of undisturbed horizontal sedimentary strata,
each stratum is younger than that below and older than that above*
Although sedimentary rocks are for the most part laid down in horizontal layers,
they may later slowly become folded and bent by earth processes. Folded layers may
become beveled by erosion and then have horizontal layers of sediment deposited on
top when the sea again covers the region. The resulting contact of nonparallel
layers is called an "angular unconformity." Obviously, those layers which truncate
are younger than those rocks which are truncated.
SLIDE 1 DIAGRAMATIC PROFILE OF THE GRAND CANYON
This diagrammatic profile or cross section of the Grand Canyon shows the relationship of the strata on the north side of the Colorado River. All of the
horizontal
layers are sedimentary* Note that each has a symbol representing its own
lithology (rock character). Also observe that the rock composing the canyon rim must be much younger than, for
example, the rock of the Tonto Platform, since it is at the top of a series of undisturbed horizontal sedimentary
strata. (Radioactive age determination indicates
the passage of more than 250 million years during the formation of these layers).
Below the rock of the Tonto Group is another series of sedimentary layers,
sloping markedly to the north and the east. This contact is an angular unconformity*
Therefore we know that the Tonto Group is younger than these sloping layers which it
truncates.
This second group rests in turn angularly upon, and is therefore younger than,
a flat erosion surface which was formed on some badly deformed rocks. These were'
once sedimentary but were squeezed and distorted in such fashion that they lost
their original character and some layers were forced nearly vertical. Since the
flat surface of erosion must have been horizontal when the next layers were deposited
upon it, but is nom, sloping parallel to the layers deposited upon it, a great
disturbance must have simultaneously affected all of the rocks below the Tonto Group.
SLIDE 2: SIX STAGES IN PRE-CA21BRIAN HISTORY
The block diagrams will explain these events and make clear the relative ages of
the rocks in question. The bottommost layers which we shall call "Rocks of the First
Era," were originally deposits of sandstone and mudstone. Great pressure from the
northwest and southeast pushed the layers up into mountains probably comparable to
the present day Alps* Heat and pressure caused the rocks to recrystallizes into a
form known as schist, and intrusions of molten rock broke through from below*
(Diagram 1).
Erosion wore down the mountains to a nearly level plain during what must have
been a vast span of time (Diagram 2).
Over this surface the sea came, causing deposition of more mud and sand. (Diagram 3).
We shall call these "Rooks of the Second Era." This time the region was
uplifted with little folding involved and these Rocks of the Second Era were broken
through together with rocks of the First Era, causing huge blocks to slide upon one
another,, forming mountains, as is shown in Diagram 4.
These mountains in turn were worn down, probably even while they were being
formed* (Diagram 5).
Once more the sea came in. The roots of the mountains of the second era were
at first islands in this sea, and then as sinking continued they finally were buried
beneath the sediment which now forms the lowest of the Rocks of the Third Era, (Diagram 6).
Continued alternate sinking and rising of base level is responsible for the rest of
the story.
SLIDE 1 again
It is not possible to give any other logical explanation of the relationship of
the strata shown in this profiles The Rocks of the Third Era must be successively
older from top to bottom, the Rocks of the Second Era must be older still, and the
Rocks of the First Era oldest of allo The strata are clearly superimposed in chronological order. Subsequent gentle doming of the whole region has enabled
the river
and its tributaries to cut the mile-deep canyon, leaving all of this history plainly
exposed to view.
in nature.
SLIDE 3: PHOTOGRAPH OF UNCONFORMITIES IN THE GRAND
CANYON
This photograph will show the clarity of the contacts as they actually appear
SLIDE 4: N-S CROSS SECTION OF
THE GRAND CANYON
Farther back on the plateau into which the Canyon is now cut there are remnants
of other layers which at one time covered the whole region. These are the Rocks of
the First Era. Their relationship to the Third Era is clearly shown by the accompanying cross section. They are flat, generally undisturbed layers and are
unquestionably younger than those of the Third Era upon which they rest conformably. On
top of them, but exposed even farther back from the Canyon, are the rocks of the
Fifth Era. Thus within an area of one day's journey strata from the First to the
Fifth Eras may be seen resting upon one another in consecutive order.
SLIDE 5: ANCIENT PLANTS AND ANIMALS
Nearly all of these layers contain some fossils. Here is an almost unparalleled
opportunity to investigate the actual order in which various types of plants and
animals are found fossilized, at least in this area.
Mother because they were actually absent or because they were destroyed in the
course of mountain building, no fossils are found in the rocks of the First Era. Thus
the first actual life is represented by colonies of fossil algae, some of the simplest
of all plant life, in the layers of the Second Era. The preservation of algae may
seem incredible, but comparison of them with modern ones shows them to be so strikingly
alike that there can be no doubt of their validity. Sponge spicules have also been
tentatively reported from the same layers and if this becomes verified, it will be of
interest that the first remnant of animal life belongs to a group of exceedingly
simple organization. Sponges are considered to be the most "primitive" of the
manycelled animals. In all of the layers from the rocks of the Second Era upward there
are found certain tubes which are most readily interpreted as worm burrows and have
been so accepted by most paleontologists.
The lowest layer belonging to the Third Era is called the Tapeats sandstone. In
this sandstone are found algae in the form of sea weed, and marine animals belonging
to two different divisions of the invertebrates (animals without backbones). one group
comprised several species of scorpion-like creatures that are now extinct, and the
other was composed of brachiopods, bivalved animals, mostly rather simple.
In the Bright Angel Shale, which lies upon the Tapeats sandstones different genera
and species of the preceeding groups are present. In addition there are other
seashells some doubtfully referred to the molluscs, others related to sea lillies, which
are really animals, a primitive crustacean or crab-like animal, and a coral-like form
of unknown affinity*
The next highest layer is the Mauv limestone and in it are Pound only different
genera and species of the animals of the preceeding shale. The Huav underwent a period
of sub-aerial erosion during which great river channels were out into it. 711hen sediment was again deposited over the region by the sea, sand and lime together with fish
remains were swept into these old channels. Later erosion removed all of what is
called the Temple Butte limestone except that which was in the river channels, and so
it is here that vertebrate life is first recorded in this secuenceo The fish were cui+e
unlike those we know in that they had a bony armour over the major portion of their
body and scales only in the hinder portions.
The Redwall limestone, resting upon the Temple Butte and Muav, contains the shells
of marine invertebrates. In addition . to many genera of braohiopods there are fragments
of sea lillies, some true colonial corals and a type of moss animal or bryozoan.
Lying above the Redwall limestone is a great thickness of alternating sandstone
and shale, the Supai Formation, which represents a delta and river flood plain environment. Here are found corals, brachiopods and true molluscs, such as clams and
cockles, but of even greater interest has been the discovery of the tracks of large
four-footed creatures which seem to have
beer- some
kind of amphibian. Reptilian type
tracks have also been found. The plant life of the time included ferns, the simplest
of land plants,
The next youngest layer, the Hermit Shale, presents one of the most interesting
array of fossils in the entire strata of the Canyon. it, too, is a land deposit,
probably the flood plain of a great river in an arid region. Here are found footprints of salamander-like animals,
wings of giant insects and 35 species of plants,
including primitive conifers or evergreens.
Immediately above the Hermit Shale is the Coconino Sandstone, a deposit of wind
blown dunes, The only traces of life in this rook are the tracks of insects and at
least 27 species
of
amphibians.
Following deposition of the Coconino sands, there was a new invasion of the sea
and in the
T.
1,aibab formation which rims the Canyon there are the remains of corals,
sponges, sea lillies and sea shells. Many of these genera also occur in the lower
layers, but in
some
cases the species of duplicate genera show a greater degree of
complexity in these upper layers. Shark's teeth are found in the Kaibab but not in
any other of the marine deposits which otherwise have somewhat similar faunal associations.
In summary, the rocks of the Third Era show first simple plant life and primitive marine invertebrate animals only, then fish remains and some different marine
invertebrates. Next are found ferns, then evergreens, and in conjunction with these
are the tracks of amphibians and reptiles and the imprint of insects. Finally there
are a host of marine invertebrates and some shark's teeth.
SLIDE 6t ROCKS OF THE FOURTH ERA
The lowermost layer of the Fourth Era is nearly lacking in fossils, but in the
Shinarump and Chinle strata above it occur the famous petrified forests. The trees
of these forests are largely conifers, but four or five other species are reported
to be present. (Unfortunately, detailed information about the plant life of all
these Bras was not accessible at the time of writing this paper). The jaw of a relative of the crocodiles has been found in one of the forests, showing that at least
one large species of amphibian lived at that time. Fresh water clam shells have been
found in association with fern impressions in this region. The seas supported a
great variety of life including many types of molluscs, some snails, cruataceans
moss animals, worms, sea lillies, corals, reptiles and fish.
It is in the overlying layers that the tracks and bones of dinosaurs are common.
Dinosaurs were true reptiles, not amphibians, and the environment in which they were
preserved for posterity as skeletons seems to have been that of river flood plains.
Their known associates were other reptiles and
some
frog-like creatures. All of the
reptiles reached quite astounding sizes before the end of the Fourth Era.
In rocks of the latter part of the Fourth Era mammal remains can be found and
there is a great decline in the number of reptilian fossils. Intercalated coal beds
indicate that there was luxuriant vegetation, and this is borne out by the presence
of bits of fossil wood and foliage of deciduous trees.
Records of life we exceedingly sparse in the Fifth Era rooks., largely limited to
fresh water clam shells which may be seen at Bryce Canyon. However, to the west of
Grand Canyon at least one deposit of bones has been found which contained elephant
tusks and camel and bison teeth, and to the southwest human artifacts have been
discovered in association with such teeth. These deposits are localized and, because
of lack of information concerning their relative age actually should not be included
within the scope of the Present survey. Nevertheless, it is certain that they are
of a later period of the Fifth Bra than that displayed at Bryce Canyon*
Thus in passing from the First to the Fifth Eras we have seen that simple plant
life and possibly animal life of low organization occur in the Second Era; that some
animals, namely the marine invertebrates, are found in one form or another from the
beginning of the Third Era, and that starting in the middle of the Third Era,
Vertebrate animals appeared successively as fish, amphibians, reptiles and mammals.
Plant life is successively recorded in algae, ferns, conifers and deciduous trees.
Possible explanations of all this and the questions which it raises cannot be
discussed here, The fact to be emphasized is simply that in the Grand Canyon and
its environs the fossil picture is one which does show a sequence from what have
been denoted "simple" forms to those which have boon denoted "complex"
Whether this
be a freak, of preservation, a more coincidence, or whether it holds deeper significance, the fact of its
existence should not be overlooked.