The Elements of Geology | Page 6

we are to learn to read.
The successive layers of the rock suggest that they were built one after
another from the bottom upward. We may be as sure that each layer
was formed before those above it as that the bottom courses of stone in
a wall were laid before the courses which rest upon them.
We have no reason to believe that the lowest layers which we see here
were the earliest ever formed. Indeed, some deep boring in the vicinity
may prove that the ledges rest upon other layers of rock which extend
downward for many hundreds of feet below the valley floor. Nor may
we conclude that the highest layers here were the latest ever laid; for
elsewhere we may find still later layers lying upon them.
A short search may find in the rock relics of animals, such as the
imprints of shells, which lived when it was deposited; and as these are
of kinds whose nearest living relatives now have their home in the sea,
we infer that it was on the flat sea floor that the sandstone was laid. Its
present position hundreds of feet above sea level proves that it has
since emerged to form part of the land; while the flatness of the beds
shows that the movement was so uniform and gentle as not to break or
strongly bend them from their original attitude.
The surface of some of these layers is ripple-marked. Hence the sand
must once have been as loose as that of shallow sea bottoms and sea
beaches to-day, which is thrown into similar ripples by movements of
the water. In some way the grains have since become cemented into
firm rock.
Note that the layers on one side of the valley agree with those on the
other, each matching the one opposite at the same level. Once they
were continuous across the valley. Where the valley now is was once a
continuous upland built of horizontal layers; the layers now show their
edges, or OUTCROP, on the valley sides because they have been cut
by the valley trench.

The rock of the ledges is crumbling away. At the foot of each step of
rock lie fragments which have fallen. Thus the valley is slowly
widening. It has been narrower in the past; it will be wider in the future.
Through the valley runs a stream. The waters of rains which have fallen
on the upper parts of the stream's basin are now on their way to the
river and the sea. Rock fragments and grains of sand creeping down the
valley slopes come within reach of the stream and are washed along by
the running water. Here and there they lodge for a time in banks of sand
and gravel, but sooner or later they are taken up again and carried on.
The grains of sand which were brought from some ancient source to
form these rocks are on their way to some new goal. As they are
washed along the rocky bed of the stream they slowly rasp and wear it
deeper. The valley will be deeper in the future; it has been less deep in
the past.
In this little valley we see slow changes now in progress. We find also
in the composition, the structure, and the attitude of the rocks, and the
land forms to which they have been sculptured, the record of a long
succession of past changes involving the origin of sand grains and their
gathering and deposit upon the bottom of some ancient sea, the
cementation of their layers into solid rock, the uplift of the rocks to
form a land surface, and, last of all, the carving of a valley in the
upland. Everywhere, in the fields, along the river, among the mountains,
by the seashore, and in the desert, we may discover slow changes now
in progress and the record of similar changes in the past. Everywhere
we may catch glimpses of a process of gradual change, which stretches
backward into the past and forward into the future, by which the forms
and structures of the face of the earth are continually built and
continually destroyed. The science which deals with this long process
is geology. Geology treats of the natural changes now taking place
upon the earth and within it, the agencies which produce them, and the
land forms and rock structures which result. It studies the changes of
the present in order to be able to read the history of the earth's changes
in the past.
The various agencies which have fashioned the face of the earth may.

be divided into two general classes. In

Part I we shall
consider those which work upon the earth from without, such as the
weather, running water, glaciers, the