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.