Geoscience Reference
In-Depth Information
weather and cause exchangeable magnesium to exceed
exchangeable calcium in the wetter soils of the catena.
Therefore soil catenas often show a chemical sequence,
with contents of more mobile and weatherable chemicals
(magnesium, manganese) increasing by migration in the
lower slope positions, and less mobile elements (iron,
titanium) increasing as residuals in the upper slope.
The second type of soil catena is as common as the first.
Not only does the solid geology commonly change down
slopes, but also hill-slope processes of erosion and
deposition cause differences in the texture and depth
of surficial deposits. In Britain during the Quaternary
period the unglaciated region south of the river Thames
experienced intense solifluction processes during the
periglacial climate, as did glaciated areas of the United
Kingdom during deglaciation. Later, human actions
leading to deforestation and cultivation from Neolithic
times onwards resulted in the accumulation of colluvial
hill wash in dry valleys and scarp-foot locations in the
English scarplands of Jurassic Limestone, Magnesian
Limestone and Chalk. A typical catena of five soils on the
Chalk scarp reflects water movement, the influences of
changes in lithology, and the effects of these hillslope
processes (
Figure 18.18
).
On slope crests and steep slopes on undulating
chalkland the soils are thin, flinty calcareous brown and
grey rendzina soils (
Plate 18.23
).
Erosion has brought the
Chalk to within 25 cm of the surface. Soil depth increases
in the scarp-foot zone with the deposition of hill wash of
silty clay loam texture in the third soil. The fourth soil is
on an outcrop of sandstone called the Upper Greensand;
this permeable, slightly alkaline soil shows clay eluvia-
tion with characteristic Eb/Bt horizon sequence. Clay
translocation in the soils of southern and eastern England
is widespread and is favoured by intense summer drying.
The final soil in this chalkland catena is a calcareous and
slightly gleyed brown calcareous soil on chalky drift; 'drift'
is a collective term which includes solifluction deposits,
hill wash and aeolian deposits. The alkaline pH reflects the
influence of ground water at this topographic position
CONCLUSION
Soils are made from rocks and sediments by a two-stage
sequence of development. Weathering produces the soil
parent material, which is then acted upon to produce the
soil profile. The particular profile formed at any point on
Earth depends upon the prevailing climate, vegetation,
rock type and topography, together with the length of time
of soil formation. Generally soils in mid and low latitudes
are deeper, more weathered, redder and less fertile than
soils of higher latitudes.
Brown Rendzina
(moderately calcareous)
Grey Rendzina
(very calcareous)
Grey Rendzina
(very calcareous)
Argillic
Brown Earth
Gleyic Brown
Calcareous Earth
0
0
0
0
0
Dark
grey
brown
c
alcareous
loam
Dark
grey
brown
clay
loam
Brown
stony
loam
Grey
brown
calcareous
loam
Brown
calcareous
clay
loam
Apk
Apk
10
10
10
10
10
Ap
Apk
Apk
20
20
20
20
20
Brown
Chalk
with
flints
Stony
loam
BCuk
Silty
loam
Cu
BCuk
Grey
brown
loam
30
30
30
30
30
Eb
Broken
Chalk
Cu
Yellow
brown
mottled
c
alcareous
clay
loam
40
40
40
40
40
Bwgk
Stony
silty
clay
loam
Chalk
R
R
Chalk
Cu
50
Yellow
brown
stony
clay
50
50
Bt
60
60
60
MIDDLE AND
LOWER CHALK
Grey
brown
stony
mottled
c
alcareous
sandy
loam
R
Chalk
70
70
Cgk
Sandstone
R
80
UPPER GREENSAND
GAULT CLAY
CHALKY
DRIFT
