ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

location, as a result of the deglaciation stage lasting for
3,000 years and the subsequent process of land uplift, and
thousands of shallow lake basins have filled in during the
last 10,000 years as a result of biomass production in the
lakes themselves and fluvial sediments entering from their
catchment areas. Meanwhile, tilting of the land surface as a
consequence of uplift has caused water levels to rise or fall
in the larger lakes in particular and their outlet channels to
alter in position. The opening up of the new outlet channel
has often been accompanied by a rapid drop in water table,
in addition to which almost 3,000 lakes have been artificially
drained and the water level in an even larger number
has been lowered.
This paper discusses the effects of natural and artificial
drops in water level and ditching in the catchment area on
sedimentation in a small lake. Changes in the ratio
between inorganic and organic material in the sediment
were examined by loss-on-ignition analysis and sedimentation
rate by pollen analysis, radiocarbon dating and spheroidal
carbonaceous particle (SCP) method. The sediment
samples were taken with a Russian corer and a Limnos
sampler. The surface part of the sediment was sliced into
subsamples of size 1 em.
The lake in question, Perhonlampi, is 0.03 km 2 in area and
has a maximum depth of 1.7 m. Its current catchment area
is 2.4 km', comprising 0.2 km 2 fields, 0.7 km 2 ditched forest
land and 1.5 km 2 forest. Two small brooks flow into the
lake, on one of which, the outlet from the ditched area, has
a large delta of mineral material at its mouth.
The lake basin was isolated from the Baltic as a result of
land uplift at the Mastogloia stage around 8000 BP, but
initially formed part of the north-westward flowing Great
Lake Saimaa complex, so that the water level in the basin
was some 12 m above that prevailing at present. When the
present outlet channel of this large transgressive lake opened
up close to the south-east border of Finland around
5000 BP, this led to an immediate drop of 2 m in the water
level. This event, which coincided with the spread of spruce
to the area is indicated by a clay stripe in the Perhonlampi
sediment.
As a consequence of a period of rapid regression, Perhonlampi
was soon isolated to form an independent basin of
its own, with a lake surface area of 0.26 km 2 and a catchment
of 10.66 km'. More than 3 m of sediment has accumulated
in the course of its existence as an independent
lake, and the proportion of organic material in this has risen
from less than 10% at the Great Lake Saimaa stage to
20-30%. In the mid-1860's most of the catchment area, i.e,
8.24 km', was linked to the adjacent drainage system via an
excavated channel, and the surface level of Perhonlampi
has been lowered artificially twice during the last hundred
years, resulting in a total drop of 2 m. Thus the lake is now
only about a tenth of its former size. The most recent lowering,
in 1964, is reflected in a fall in loss on ignition values
in the sediment.
The greatest change in the sedimentology of the lake took
place in 1971, however, when extensive ditching was carried
out in the forests of the catchment area. This led to
the creation of a delta in the lake, brought about by fluvial
erosion, and the lake bottom is now completely covered by
a layer of silty material of average thickness 15 em. Organic
matter drops from over 25 % to 5-10 % at this point in
the sediment, accompanied by a distinct rise in the numbers
of SCP in spite of the rapid rate of sedimentation, an
obvious indication of the sharp increase in the use of fossil
fuels in Finland in the 1970's.
A total of some 4,200 m' of silt has accumulated in the lake
since the ditching operation, with an average deposition
rate of 11 mm a year, as compared with an average rate of
0.67 mm a year over the last 5,000 years. The results are a
clear reminder that human activity may have a decisive impact
not only on the quality of lake water and sediments
but also on the existence and lifetime of lake basins. If the
sedimentation rate in the basin examined here remains at
the current level, the lake will fill up and disapper in some
150 years' time, whereas this would have taken 6,000 years
in the natural course of events, and some 2,500 years even
if the water level had been lowered artificially.
SERGEI N. TITKOV
Investigations of rock glaciers of the Northern Tien Shan
Permafrost Institute, p.o. box 138, 480001, Almaty, Kazakhstan
There are 1,034 rock glaciers in two parallel latitudinal
ranges of the Northern Tien Shan - Zailiysky Alatau and
Kungei Ala Too (42,5-43,5 N, 76-79 E). The highest point
of two ranges is the Talgar peak (4973 m a.s.l.). The central
part of these ranges is strongly glaciated: 880 glaciers
of total area about 855 sq. km occur here.
There are 1,034 rock glaciers in two ranges. On the
morphological signs, 851 of this amount of total area 90,28
sq. km are considered to be active (Titkov, 1988). The best
studied rock glaciers are situated in the central part of
northern slope of the Zailiysky Alatau in basins of rivers
Bolshaya and Malaya Almatinka.
The investigations of rock glaciers of the Northern Tien
Shan started in 1923 by geodetic observations of russian
glaciologist N.N.Palgov near the front of Gorodetsky rock
glacier. Basing on his geodetic net, the observations were
repeated six times till 1994. Additional data on the temporal
variations of movement have been obtained resently by
the use of aerial photografs taken in different years (Gorbunov
& alii, 1992).
Over a 71 year period, mean displacement of the central
part of the frontal scarp of the rock glacier was 63 m or
0,93 m/yr. The rate of movement of lateral parts of the
front did not exeed 0,18-0,23 m/yr. Maximum velocity of
surface movement reached 1,3-1,5 mlyr in the middle part
of rock glacier about 150-200 m from the front. This difference
between rates of movement of surface and front results
in formation on the surface of rock glacier transverce
ridges and furrows as well as lobes up to 8 m high.
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