Every geologist is familiar with the erosion cycle. No
sooner has an area of land been raised alive sea-level than it becomes subject
to the erosive forces of nature. The rain beats down on the ground and washed
(51) the finer particles, sweeping them into rivulets and then
into rivers and out to sea. The frost freezes the rain water in cracks of the
rocks and breaks (52) even the hardest of the constituents of
the earth’s crust. Blocks of rock dislodged at high levels are brought down by
the force of gravity. Alternate heating and (53) of bare rock
surfaces causes their disintegration. In the dry regions of the world the wind
is a powerful force in removing material from one area to another. All this is
natural. But nature has also provided certain defensive forces. Bare rock
surfaces are in (54) course protected by soil itself
dependent initially on the weathering of the rocks. Slowly (55)
surely, different types of soil with differing "profiles" evolve the
main types depending primarily on the climate. The protective soil covering,
once it is formed, is held together by the growth of vegetation. Grass and
herbaceous plants, (56) long, branching tenuous roots, hold
firmly together the surface particles. The (57) is true with
the forest cover. The heaviest tropical down- pours beating on the leave of the
giant trees reach the ground only (58) spray, gently watering
the surface layers and penetrating along the long passages provided by the roots
to the lower levels of the soil. The soil, thus protected by grass, herb, or
trees, furnishes a quiet habitat for a myriad varied organisms-earth-worms that
importantly modify the soil, bacteria, active in their work of converting
(59) leaves and decaying vegetation into humus and food for the
growing plants. Chemical action is constantly taking (60) .
Soil acids attack mineral particles and salts in solution move from one layer in
the soil to another.