Geology Reference
In-Depth Information
Karst windows
()Border polje
a
These are unroofed portions of underground caverns in
which streams flow out of the cavern at one end, across
the floor, and into a cavern at the other end. The openings
may be mere peepholes or much larger.
Uvalas and egg-box topography
( ) Structural polje
b
Uvalas
, a word from Slovenia, are compound sinkholes
or complex depressions composed of more than one hol-
low. They are larger than small dolines. Elongated forms
follow strike lines or fault lines, while lobate forms occur
on horizontal beds. Solution may play a big role in their
formation, but, without further study, other processes
cannot be discounted.
On thick limestone, where the water table is deep,
solutional sinkholes may be punched downwards to form
egg-box topography
, known as
fengcong
in China, with
sharp residual peaks along the doline rim and a local relief
of hundreds of metres.
()Baselevel polje
c
High/low water table
Polja
Permanent flow
Intermittent flow
A
polje
(plural
polja
) is a large, usually elongated, closed
depression with a flat floor (Plate 8.6). Polja have many
regional names, including
plans
in Provence, France;
wangs
in Malaysia; and
hojos
in Cuba. Intermittent or
perennial streams, which may be liable to flood and
become lakes, may flow across their floors and drain
underground through stream-sinks called
ponors
or
through gorges cutting through one of the polje walls.
The floods occur because the ponors cannot carry the
water away fast enough. Many of the lakes are seasonal,
but some are permanent features of polje floors, as in
Cerkni
Figure 8.9
Types of polje. (a) Border polje. (b) Structural
polje. (c) Baselevel polje.
Source:
After Ford and Williams (1989, 429)
rocks in limestone terrain. They include the largest karst
depressions in the world and are the dominant type of
polje in the Dinaric karst.
Baselevel polja
occur in lime-
stone where a regional water table intersects the ground
surface.
ˇ
ca Polje, Slovenia.
Polja come in three basic kinds: border polja, structural
polja, and baselevel polja (Figure 8.9) (Ford and Williams
1989, 431-2).
Border polja
are fed by rivers from out-
side the karst region (allogenic rivers) that, owing to the
position of the water table in the feed area and flood-
plain deposits over the limestone, tend to stay on the
ground surface to cause lateral planation and alluviation.
Structural polja
are largely controlled by geology, often
being associated with down-faulted inliers of impervious
Cone karst
Tropical karst is one of the landform wonders of the
world. Extensive areas of it occur in southern Mexico,
Central America, the Caribbean, South-East Asia, south-
ern China, South America, Madagascar, the Middle East,
New Guinea, and northern Australia. Under humid trop-
ical climates, karst landscapes take on a rather different
aspect from 'classic' karst. In many places, owing to
rapid and vigorous solution, dolines have grown large
