Page 175 - Life & Land Use on the Bahrain Islands (Curtis E Larsen)

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necessary to consider two broad morphological regions and their influence on
stream development. In the broadest sense, these are the coastal plain and the
adjoining slopes of the Bahrain dome; and the interior basin within the central
portion of the dome. These represent vastly different drainage characteristics that
have influenced sedimentation.
Tbe first of these morphological regions is marked by a radial drainage
pattern with wadis and gullies directed down the slopes of the central dome,
following the dip of the Khobar limestone. Such wadi channels are incised into the
underlying carbonate rocks in places, but are most often bounded by partially
consolidated deposits of probable Pleistocene age. These range from silts to
angular and subangular rock fragments formed by weathering of the underlying
limestone and dolomite. The main channels to these wadis vary in length from 1 to
4 kilometers depending upon the portion of the slope they transect. Shorter length
wadis occur on the east and west flanks of the dome where beds are more steeply
dipping. On the northern and soutern flanks of the dome lower dips allow broader
surface exposures of the Khobar limestone which are crossed, in turn, by wadis of
greater length.

Tbe weathered surfaces of the dome supply abundant weathering debris
which is carried downslope where it accumulates in broad coalescent fans of fluvial
and colluvial material. These fans mark the intersection of the Khobar dipslope
with the coastal plain where stream gradients flatten abruptly and where runoff is
no longer confined within defined channels. Present wadis tend to form braided
channels at this interface as runoff spreads across the fans leaving a thin veneer of
debris across the surface. Along the western margin of the dome, such fans
interdigitate with earlier Pleistocene marine deposits and eolian sediments carried
inland by the prevailing onshore winds from the northwest. Thus the area is
periodically supplied with fresh sediment as intermittent rainfall carries weathered
debris downslope.
A representative channel along the northwestern slope of the dome is 2
km long and increases in width from 2 m at its head to 12 m at its mouth, the point
of debouchment onto the coastal plain. Unconsolidated sediments exposed along

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