A Note on the Geomorphology of the Country near Umm Dabaghiyah

Abstract

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 geomorphological 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