If in some part of North America there had been steady accumulation of
sedimentary materials under constantly favorable conditions since the
beginning of Cambrian time, the result would have been a deposit of
sandstones, claystones, and limestones measuring nearly fifty miles from
bottom to top. These figures are based on actual production in North
America where extensive measurements have been made in many localities.
When other parts of the world are as thoroughly investigated and older
deposits included in the calculations, the total thickness of such beds
will probably be more than one hundred miles.
No single pile of rocks offering a complete cross section of the
geological record has ever been produced, but portions of the section
are exposed to view on all the continents. In order to carry on
desirable investigations and make comparisons, it has been necessary to
divide this great composite section into small units which may be named
in some way and placed definitely with relation to lower and higher, or
older and younger, layers. To serve this purpose there has been
developed the idea of rock _formations_, and here we have a word which
is not defined readily, even for the use of those who are familiar with
it. Nevertheless it is used so commonly that some understanding of its
meaning becomes desirable.
A _formation_ may be regarded as an extensive rock mass, variable, in
thickness and other proportions, as well as in composition, but
representing a period of time during which there was no great change in
the character of plant and animal life, and no serious interruption in
the depositing of the rock-making materials. Occasionally the lower and
upper limits of a formation are well defined and readily located.
Frequently, however, the transition is gradual, one formation merging
into another with no apparent mark of separation. In such event the
original description serves to establish more or less definitely the
boundaries of a formation.
Descriptions are published whenever a worker believes he has discovered
a significant part of the great section which has not previously been
named. The usual practice is to apply a name taken from the locality in
which the beds were investigated, and in this manner the names of
formations become associated with towns, rivers, counties, mountains,
states and other geographical features. The locality which supplies the
name is then regarded as the “type locality” for the formation, but
wherever these same beds may be traced or otherwise identified the one
formation name applies.
[Illustration: Dinosaur Tracks
An ancient trail in sandstone of the Dakota formation. East slope of
the hogback, west of Denver.]
The “Dakota formation,” to use a convenient illustration, is mentioned
in scores of reports bearing on the geology of Colorado, Iowa, Kansas,
Nebraska, New Mexico, Texas, Utah, and Wyoming, as well as the Dakotas.
On the geological map of Colorado it appears on both sides of the
Rockies, scattered in strips and patches from north to south boundary
lines. The beds are easily located in the foothills district west of
Denver because of their tendency to produce the prominent ridges known
as hogbacks.
Many formations are exposed over much less territory, some have even
greater extent. Thickness may vary from a few inches to thousands of
feet, and no two exposures will be exactly alike though some similarity
necessarily prevails throughout. “Exposures” are simply portions of the
beds which are not concealed by loose rock, soil and vegetation, or
overlying formations. Canyon walls, steep cliffs and mountain slopes,
gullies, and badlands provide a large variety of natural exposures. In
such places rocks and fossils may be studied to best advantage.
Since a formation may contain a variety of beds, including sandstones,
shales, limestones, and all sorts of mixtures, there is sometimes need
of subdividing it; but formations are the smallest units commonly shown
on geological maps. They are actual rocks which fit into a historical
scheme of things and may be regarded aptly as the pages of a book which
nature has done in stone.