254
Climatic Geomorphology
Figure 11.12. Biocrust developed on Miocene clays and superficial colonized by lichens. Observe their
major resistance to erosion. Experimental station of Lanaja Ebro Depression, Huesca Province, Spain.
a result of joint physical and chemical processes (Ollier, 1984; Cooke et al., 1993). The
expansion and contraction of bacteria and lichens, in relation to water content variations,
originate the rock disintegration by granular disintegration and exfoliation in surface as
well as in confined spaces. Chemical processes come out by the secretion of organic acids,
which attack the rock constituents and generate chelates that help to export cations of low
solubility. The carbon dioxide of the microbiological activity favours the dissolution of the
limestone surfaces, originating in biological microkarren, characterized by the presence of
tiny furrows and hollows (Ollier, 1984). Finally, some of these micro-organisms play a
fundamental role in the genesis of the desert varnish.
Mobile dune
,,/
Mobile dune
~, 1~
~~
North-facing
'./:-":i';:'i~i':~'.~':"~''-..~ V~., slope
South-facing ~i:!~.-~ii:~i:i-i:.~;~!ii!
".:.':"-'-" :'.'" ." "'"
,
slope ;,-."'~" :.';~'-:-"!'- ";'-' :-!:
"!':~ "'f-::i: ~"]." ".1---".'.'.~~-"
Interdune
./'S:. )~!'=,'!ii "~ .~ i')- :'i!
.:" "~-~-:} ~ .'.. ",~
Dune
/~".".
!! 5
9 ." "--~
:'-" !..'..-."~_
,
area
.
/~~."
:-~.
9 - : ~.--1
"- .~ ".~.
base
,#l~..'.. v..: :.';:
9 . ,.:;...:
9 . :
tk
..'.
....
: ., .....'."
::
SS";"~::!);~:: ":
,J/x,. PlaTa ,~,fi,
'
:
Figure 11.13. Transversal cross section of the longitudinal dunes system in the experimental station of
Nizzana, Negev Desert, Israel (Veste, 1995).