Geologically, our search area belonged to the onshore portion of the Aaiun Basin that extends for about 680 miles from the Cap Blanc Fracture Zone in northern Mauritania north through southern Morocco to the intersection of the North Canary Island Fracture Zone and the South Atlas Fault.
In the east, some 170 miles from the Atlantic coast, it is separated from the older Tindouf Basin by a swell in the northern extension of the Precambrian Reguibat Massif and Palaeozoic fold belt of the Mauritanides. The Aaiun Basin comprises Mesozoic and Cenozoic continental and shallow marine sediments overlying a basement of Precambrian and Palaeozoic age.
The surfaces we were dealing with were Cretaceous, Tertiary and Quaternary sediments deposited in the Aaiun Basin, with the top horizons consisting of Pleistocene pediments composed of weathered limestone fragments and quartzite pebbles mixed with cherts, silcretes, sandstones, shales and dolomitic carbonates, often overlying a fine-grained silt. The varying textures, colours and respective degree of desert varnish on the surface accounted for a chaotic pattern that made the visual identification of meteorites challenging.
Because the general topography followed the north-to-south orientation of the Aaiun Basin, moving transversely to the orientation of the elevation profile, as we did, brought successive changes of the surface pattern with it. Thus, travelling west-to-east required re-adjusting to different types of background rocks every ten or fifteen kilometers. One half of the team was already used to these difficult search grounds from the previous trip to the Hammadas, north and south of the Saquia al Hamra. There, two years ago on occasion of our first expedition in the area, we had proven that even on surfaces offering little-to-no background contrast to the commonly dark brown or black meteorites, finds could be made, albeit only after a lot of time and effort.
Andi and I stopped frequently in order to pick up dark rocks and get acquainted with the local pediment surface and the meteorwrongs among it. It would take us another few days until a casual glance from the corner of the eye would be sufficient to recognize the black chert gravel as such and exclude them as targets. Many more hours of searching would be necessary to develop the unerring security in our assessment that was necessary to pick out one promising target among tens of thousands of rocks passing by in an average search hour.
Given their vast experience from meteorite prospecting in other deserts, I trusted that the others would quickly adapt to the difficult search grounds. In particular Rainer was a skilled mineralogist and seasoned field expert, and we were glad to have him with us since his judgements could be utterly trusted.