Page 217 - Fundamentals of Geomorphology

200 PROCESS AND FORM

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
b
( ) Structural polje
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, () Baselevel polje
c
solutional sinkholes may be punched downwards to form
High/low water table
egg-box topography, known as fengcong in China, with
sharp residual peaks along the doline rim and a local relief
of hundreds of metres.

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 Figure 8.9 Types of polje. (a) Border polje. (b) Structural
regional names, including plans in Provence, France; polje. (c) Baselevel polje.
Source: After Ford and Williams (1989, 429)
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 rocks in limestone terrain. They include the largest karst
underground through stream-sinks called ponors or depressions in the world and are the dominant type of
through gorges cutting through one of the polje walls. polje in the Dinaric karst. Baselevel polja occur in lime-
The floods occur because the ponors cannot carry the stone where a regional water table intersects the ground
water away fast enough. Many of the lakes are seasonal, surface.
but some are permanent features of polje floors, as in
Cerkniˇca Polje, Slovenia. Cone karst
Poljacomeinthree basic kinds: border polja,structural
polja, and baselevel polja (Figure 8.9) (Ford andWilliams Tropical karst is one of the landform wonders of the
1989, 431–2). Border polja are fed by rivers from out- world. Extensive areas of it occur in southern Mexico,
side the karst region (allogenic rivers) that, owing to the Central America, the Caribbean, South-East Asia, south-
position of the water table in the feed area and flood- ern China, South America, Madagascar, the Middle East,
plain deposits over the limestone, tend to stay on the New Guinea, and northern Australia. Under humid trop-
ground surface to cause lateral planation and alluviation. ical climates, karst landscapes take on a rather different
Structural polja are largely controlled by geology, often aspect from ‘classic’ karst. In many places, owing to
being associated with down-faulted inliers of impervious rapid and vigorous solution, dolines have grown large