Factors of Pedogenesis - Major Soil Groups of the World - page 37

Table 2.3 Approximate time-scales in pedology (time necessary for obtaining
a soil belonging to the type indicated).

Time in years Degree of development Examples

1000 No visible change in slowly Alluvial and colluvial soils

soluble mineral elements

3000-5000 Rapidly differentiated mature Podzol

soil

50,000-100,000 Slowly differentiated mature soil Luvisol

100,000-500,000 Degraded soil Planosol

10 to 50 million Ancient residues Ferrallitic cuirasses

constitute a chronosequence used for tracking pedological evolution, step
by step. The most typical chronosequences of soils are established on
fluvial terraces, glacial moraines, marine terraces and offshore bars.

2.3.2 Chronosequences on Fluvial Terraces
Structure

Under certain circumstances, streams do not hollow out their beds,
but meander, deposit their alluvial loads and form a flat-bottomed
valley. Then, for reasons we will discuss below, they locally downcut
into their own materials before starting again to meander and deposit
alluvia. Often, the second valley bottom is narrower than the preceding
one of which traces remain on the sides in the form of a stair called
terrace . In some cases the erosion phases follow each other and are not
separated by large phases of alluviation (Fig. 2.7a). The levels are more
and more recent as we move down. On the other hand, if there is
banking up (Fig. 2.7b), a recent deposit might be higher than an older
deposit observed in section, but this is not important: if one stands
on terraces observable on the surface, the chronology persists and the
highest terrace is the oldest.

a: without an intermediate phase of
filling in

b: with an intermediate phase of
filling in

1

First phase of cutting; establishment
of the oldest soils

4

Fill

3

2

Terraces

The terraces are formed in the order 1,
2, 3, 4; only 1 and 4 are seen

Second phase of
cutting; establishment
of soils of medium age

Third phase:
establishment of
the youngest soils

Fig. 2.7 Section through valleys showing the formation of fl uvial terraces.