A Note on the Geomorphology of the Country near Umm Dabaghiyah Author(s): Peter Dorrell Source: Iraq, Vol. 34, No. 1 (Spring, 1972), pp. 69-72 Published by: British Institute for the Study of Iraq Stable URL: http://www.jstor.org/stable/4199932 . Accessed: 12/06/2014 21:06 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . British Institute for the Study of Iraq is collaborating with JSTOR to digitize, preserve and extend access to Iraq. http://www.jstor.org This content downloaded from 62.122.79.52 on Thu, 12 Jun 2014 21:06:38 PM All use subject to JSTOR Terms and Conditions
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A Note on the Geomorphology of the Country near Umm DabaghiyahAuthor(s): Peter DorrellSource: Iraq, Vol. 34, No. 1 (Spring, 1972), pp. 69-72Published by: British Institute for the Study of IraqStable URL: http://www.jstor.org/stable/4199932 .
Accessed: 12/06/2014 21:06
Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp
.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].
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A NOTE ON THE GEOMORPHOLOGY OF THE COUNTRY NEAR UMM DABAGHIYAH
By PETER DORRELL
The exceptional weather during much of the 1971 season at Umm Dabaghiyah provided an unusual opportunity to observe the steppe-country of the Jazira in flood conditions. Series of violent though short-lived storms produced an effect on the land surface which was the more pronounced because of the drought, and consequent de-vegetation, of the previous twelve months. The difficulties of travel in the water-logged countryside in the short time available prevented any systematic survey, nor was the party equipped for geomorphological measurements, but even the limited observations possible revealed much of interest, both from the point of view of present-day land-forming processes, and concerning the possible geo- morphological history of the area.
Umm Dabaghiyah lies within the dry steppe zone of the Jazira. The Lower Fars series dips gently to the NNW, and the surface levels consist of redeposited silts and clays, more or less cemented by gypsum, lying upon gypsiferous rocks and gravel. The topography is of shallow valleys between low, smooth hills, often with rock outcrops near their summits. The differences in elevation are usually only of some few metres. There appears to be no coherent system of drainage within the area; storm-water drains from the hills in the form of sheetwash to form playa-lakes in the lowest-lying valleys. Only exceptionally are there signs of a permanent system of linear wadis debouching into the valleys or connecting one playa with the next. Presumably this lack of incised drainage results from the small differences in elevation and the consequent low energy potential of the system.
Along the Wadi Tharthar, which flows north to south immediately east of Hatra, linear tributary wadis do feed into the main channel, but the system extends only a short distance laterally. The pattern of unconnected playas starts a few kilometres west of Hatra and continues far into the Jazira. The Tharthar near Hatra flows in a steep-sided valley some 50 m. below the surrounding surface, and within this valley it is now cutting through a terrace another 6 to 8 m. thick. This well-marked step, and its incision, may have resulted from tectonic subsidence, but it is tempting to equate them with the Older Valley Fill and its erosion, a sequence well-attested in the Mediterranean and in other parts of the Near East.' If the lack of an extensive lateral drainage system into the Tharthar is not solely due to the Wadi's recent formation, two other explanations might be considered. It may be that rainfall is, and in recent geological times has always been, so low and intermittent as to preclude the formation of such a system; or that surface permeability is so high that most surface water drains subterraneously; or, of course, a combination of the two explanations. Even in overcast weather the level of some playas could be seen to fall by tens of centimetres a day, indicating a considerable lateral movement of
1 C. Vita-Finzi, 77te Mediterranean Valleys (Cambridge, I969).
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sub-surface water, although such movement may be a matter of local adjustment of levels rather than of general directional flow. Present-day rainfall is below 200 mm. per annum, with a strongly seasonal distribution, and there is little geo- morphological evidence for its having been any greater in the historical past.
During the few weeks of the 1971 season, however, one of the most striking and obvious consequences of the series of storms was the widespread cutting of small scale linear erosion channels, both where run-off was concentrated by the topography and where small breaks in the ground surface provided initial nicks (Plate XXa, b). Although the year's weather pattern was exceptional it can hardly have been unique (local opinion is that such seasons occur every twenty to thirty years) and it is surprising that no system of even local incised channels has been perpetuated from previous storm seasons. An explanation might be the occurrence of dry-season dust-storms which would tend to fill in channels and to smooth the country generally, but any such fillings could be expected to remain relatively unconsolidated and the channels to be quickly re-excavated by subsequent run-off; incision of the solid would then continue. The evidence seems rather to indicate a situation in which linear erosion had not previously occurred for a very long time. Factors of vegetation and perhaps alterations in the seasonal distribution of rainfall might have played a part in this change of regime, but a more likely explanation lies in recent changes in land-use.
The enormous expansion of mechanized agriculture in the Jazira and in the north Syrian plain in the early I960's apparently reached as far south as Umm Dabaghiyah, and for some seasons the steppe country was ploughed for cereal cultivation. A few years ago the government became concerned about the resultant loss of top-soil through blowing, and fixed the southern ploughing limit north of Umm Dabaghiyah. Changes in vegetation and the breaking and pulverization of surface crusts due to these activities,, together with the partial deflation of soil material, may be at least in part responsible for the emergence of an apparently new erosional regime.
At present the region has a sparse covering of hamal evergreen, its density varying with run-off concentration. In more normal years there is also a discontinuous seasonal cover of grasses and small annuals. Individual plants stand with their stalks and upper roots in a cone of soil material, much of it probably trapped wind- blown or saltated sediments. It is noticeable that as the ground surface dries after rain these cones, and in fact the area immediately around any plant, dry more quickly than do unvegetated patches (Plate XXIa, b). A part of this accelerated drying is no doubt due to absorption of moisture by plant roots, but the fact that it occurs no matter how frequent the storms, when, presumably the plants' absorption has reached capacity, indicates that a part of the drying at least is due to faster drainage through the root-disturbed soil. Unvegetated areas are smoother-surfaced, more compacted, and presumably less permeable. During sheetwash the surfaces of such areas are eroded by a process akin to etching-the picking-out of lines of surface weakness, often the bounding cracks of contraction polygons, and the quarrying and comminution of very small blocks and surface flakes.
A trial pit was excavated a short distance from the tell and revealed a strongly developed concretionary gypsum hardpan some 50-70 cm. below the surface and
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at least IOO cm. thick. From the evidence of track-cuttings and similar chance exposures this layer is present over a wide area, and in places erosion of super- incumbent levels has left the pan as a surface crust. Although not impermeable (water-holes at a nearby deserted farmhouse were cut through this pan to underlying gravels) this layer must certainly affect the vertical and lateral movement of ground- water and at least hinder the growth of tree roots-the whole region is, with a very few exceptions, treeless. It is not easy to make a reliable estimate of the date of the formation of this level. In the very limited area of pre-occupation surface revealed at the end of the dig, it appeared to continue beneath the tell, but this is not con- clusive evidence of its having been formed in pre-tell times, since its position might be connected more with the level of some seasonal water-table than with the immediate contours of the ground, and the tell is perhaps too small, and the capillarity of its material too low, to affect the local water-table very much. Such gypsum layers are widespread in northern Mesopotamia and Assyria in areas of similar lithology and structure.2 They are thought to result both from the evapora- tion of saline groundwater and from precipitation within the groundwater horizon itself." It is uncertain whether they are necessarily a consequence of human activity, or whether they may have been formed by a combination of climatic, pedological and perhaps vegetational circumstances quite apart from man's influence. Certainly there is no reason to think that they were all formed simultaneously, or that their formation was anything but a long-term process.
In one section of the trial pit the pan had invaded and fossilized an ants' nest, infilling the runs with crystalline gypsum. This would confirm, at least, that the pan is a secondary formation, and the depth of the nest below the surface-about 6o cm.-is of the same order as active present-day nests. This evidence is not conclusive because the depths to which ants excavate their nests varies from species to species and is often determined by the depth of the water-table, but it does at least suggest that the surface has been neither greatly denuded nor greatly alluviated since the nest was in use. If this were so it would in turn indicate that a non-linear erosion pattern had persisted for a very long time, since, had there been a fully linear system, the relic wadis would have remained, which they do not, or could only have been eliminated by a considerable lowering of the land-surface, which apparently has not occurred.
Inexact as these observations are, they do lead to a few, very tentative conclisions, and may point the way for future field-work. In the first place it seems likely that no major climatic change of a sort to leave its imprint on the landscape has occurred in this area during historic or recent prehistoric times. Secondly the delicate balance of climate, vegetation, soils and land-forms often observed in other semi-arid regions is equally present in the Jazira. Apart from climate which is largely independent of local causes, these factors have a considerable degree of interdependence, and no great change in one or the other could have occurred without affecting the rest, nor, probably, without altering the optimum or indeed possible land-use and the
JR 7
' H. M. Yahia, Soils and Soil Conditions in Sediments of the Ramadi Province (Iraq) (Amsterdam, 197 1).
a V. A. Kovda, in Proc. of the Teheran Symposium, Arid Zone Research No. 14 (UNESCO, Paris, I961).
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area's potential for settlement. Changes in space or time of economic bases, settle- ment density and size of social units need not, therefore, reflect any major environ- mental changes, still less the incursions of new groups of peoples, but rather the development or selection of optimum methods of land-use from among a number of possibilities. In particular a change in the seasonal rainfall distribution, in the tendency for one sort of weather to follow another, or in the tendency of the land- surface because of its underlying structure or because of vegetation, to affect run-off, absorption and retention of water might profoundly affect land-use and settlement.
One other possibility is suggested by the climatic regime and its effects. Although the exceptional weather of I971 occurred in one form or another throughout the Middle East, in more normal times the rainfall pattern of the Jazira is extremely local. Small depressions follow curved paths approximately from NW. to SE. during the rainy seasons. These paths, and the rainfall totals, vary from one season to another and areas of country cannot therefore be guaranteed to support agriculture, or even stock-keeping, with any sort of reliability. If political units were small, and social and economic connections local, this unreliability might greatly restrict settlement. A political unit covering a large region, however, would com- mand a resource base far greater than the sum of its component parts, since shortages in one area would almost certainly be compensated for by surpluses in another, and by quite short-distance movements of people from one season to another, a considerably larger population could be maintained than would have been possible otherwise. Whether and how far this enlargement of a regional economic base occurred with the growth of early empires can only be discovered by archaeological and historical studies, but at least it seems likely that the environment allowed of the possibility.
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