s its highest point and begins to ebb. Canals lying parallel to
its course are filled with water which is saved for use in the hot, dry
summer. As the flood goes down, a deposit of slimy mud lies as a rich
fertilizer on the land. It is this and the water which the earth has
absorbed that make Egypt one of the most fertile agricultural countries
in the world.

The region covered by the Nile's overflow is the flood plain of this
river. On this plain the Pyramids, the Sphinx, and other famous
monuments of Egypt stand. The statue of Rameses II. built 3,000 years
ago, has its base buried nine feet deep in the rich soil made of Nile
sediment. A well dug in this region goes through forty feet of this soil
before striking the underlying sand. How many years ago did the first
Nile overflow take place? We may begin our calculation by finding out
the average yearly deposit. It is a slow process that accumulates but
nine feet in 3,000 years. If you were in Egypt when the Nile went back
into its banks, you would see that the scum it leaves in a single
overflow adds not a great deal to the thickness of the soil. Possibly
floods have varied in their deposits from year to year, so that any
calculation of the time it took to build that forty feet of surface soil
must be but a rough estimate. This much we know: it has been an
uninterrupted process which has taken place within the present
geological epoch, "the Age of Man."

Not all the rich sediment the Nile brings down is left on the level
flood plain along its course. A vast quantity is dumped at the river's
mouth, where the tides of the Mediterranean check the river's current.
Thus the great delta is formed. The broad river splits into many mouths
that spread out like a fan and build higher and broader each year the
mud-banks between the streams. Upper Egypt consists of river swamps.
Lower Egypt, from Cairo to the sea, is the delta built by the river
itself on sea bottom. From the head of the delta, where the river
commences to divide, to the sea, is an area of 10,000 square miles made
out of material contributed by upper Egypt, and built by the river.
Layer upon layer, it is constantly forming, but most rapidly during the
season of floods.

Coming closer home, let us look at the map of the Mississippi Valley.
Begin as far north as St. Louis. For the rest of its course the
Mississippi River flows through a widening plain of swamp land, flooded
in rainy seasons. Through this swampy