ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

In relation with the existence of this weathering crust, the
epikarst in gypsum is not well developed. However local
situations exist with absorbing cavities both inside bubbles
and pressure ridges. An interesting aspect of this epikarst
is that while most of the pre-existing discontinuities are
sealed near to the surface they tend to· be open at the
depth of a few meters. This aspect is in accordance with
the model of volume increasing of the outer layer.
The causes of this changing of the characters of the outer
rock layers are not jet well explained. The explanation of
most of the Authors that they are the result of the transformation
of anhydrite in gypsum are probably simplicistic.
The mineralogical analyses carried on in some of the studied
areas reveal that the anhydrite is present only in a very
small amount.
Between the other processes that could explain the increase
in volume we list: the transformation gypsum - bassanite,
the recrystallization of gypsum (perhaps also in connection
with the annual cycle of the porosity water), an increase
of porosity in consequence of the tensional relaxation,
the phenomena of thermal dilatation, a plastic deformation
linked with a rearrangement of crystal's location, a combination
of more different processes.
We are planning specific research works to clarify the more
effective processes involved in this peculiar morphological
evolution.
MARIA 1. MACHADO 1, ALFREDO PEREZ-GONZALEZ 2
& GERARDO BENITO 1
Erosion processes and land degradation episodes
during the last 3,000 yr at the Axum Region
(Tigray, Northern Ethiopia)
1 Centro de Ciencias Medioambientales, Csic,
Serrano 115 bis, 28006 Madrid, Spain
2 Facultad de Geologia, Univ. Complutense de Madrid,
28040 Madrid, Spain
The region of Axum, in the province of Tigray, Northern
Ethiopia, is characterized by a long established agricultural
background, considered to be one of the oldest in the African
Continent (over 2,000 years). The effects of such human
disturbance in combination with other external environmental
stressors, such as climatic variability, can be
found in slope deposits and in particular at tributary infilled
valleys, where sequences of coarse materials are interbedded
with buried soils.
The study of land degradation processes and its causes involved
reconstructing the human and natural environment
during the late Holocene, using mainly geomorphological
and geoarcheological techniques.
Stratigraphical analysis of several infill valleys enabled the
identification and characterization of three main land degradation
episodes, the oldest one dating 2250±190 yr BP,
258
corresponding to the first large scale human intervention in
the area: introduction of new technologies such as the irontipped
plow, and new plants such as barley and wheat. Regarding
the other two episodes, the first one can be attributed
to the declining period of the Axumite Kingdom (ninth
to eleven centuries), whereas the second one was particularly
active during the eighteen-nineteen centuries.
During the last five to six decades, geomorphological evidences
reveal a sudden increase on erosional activity, affecting
mostly slopes, and in the last 30 years shifting towards
the valley floors, producing deep incisions at the infill valley
deposits and channel metamorphosis. During this period
two important events can be distinguished: the first,
large-scale deforestation of 1936-39 and the changes on
land tenure and property size, produced after the 1975 rural
reform.
MARY ANN MADEJ
The development of longitudinal channel bed patterns
U.S. Geological Survey Redwood Field Station, 1125 16th St. Room 207,
Arcata, CA 95521, USA
Following large floods and large inputs of sediment in
1975, the channel bed of Redwood Creek became almost
flat and featureless. The river thalweg was surveyed in three
reaches in 1977, 1983, 1986, and 1995. One reach degraded
from 1977 to 1995, one aggraded, and one remained
about the same. In 1977, variation in bed elevations
was low and channel bed elevations showed little periodicity.
During the subsequent 20 years, channel bed elevations
show increased variance and the channel bed has developed
patterns of distinct periodicity, with shorter wavelengths
through time. The development of these bed patterns
in a gravel bed river contributes to channel roughness.
Roughness in gravel bed rivers is generally defined
only in terms of a characteristic grain size, but larger scale
roughness factors (bed clusters, gravel sheets, bars, pools
and riffles) also contribute to flow resistance.
This study used a statistical analysis of series of bed elevations
as a method to quantify changes in bed roughness
and bed pattern over time. The hypothesis tested was that
a series of bed elevations is random after large sediment inputs
and becomes both more variable and more systematic
through time. However, longitudinal patterns in Redwood
Creek are neither perfectly random nor perfectly regular
because of the interaction of random processes (i.e., tree
falls causing pools) with processes producing periodic patterns
(such as alternate bar formation).
Channel widths in the study reaches vary from 60 to 110
m, median particle size D 50 ranges from 15 to 32 mm. The
river has low sinuosity (1.03 to 1.10), with an alternate bar
planform. Typical bar lengths are 500 to 650 m.