With sizes between 4 to 40 centimeters, shaped into typical meteorite forms by wind corrasion and coated with the velvety glaze of desert varnish, the dark silcrete fragments were practically indistinguishable from an unweathered fusion crusted meteorite. Nevertheless we gave it a try, but soon found that the rocks were simply too abundant to diligently check even a fraction of them and so we had to simply give up.
The fact that these dark silcretes were evenly distributed on the surface with wide enough spaces between to not form a single monochrome blanket, provided a worst case scenario that rendered any attempt to prospect for meteorites in this area pointless. We dubbed the silcrete field ‘The black pest” and swung around, in order to carry out another traverse to the south, parallel to the determined axis of our predicted strewnfield.
Later that day, Andi, who was the last of us without a find of his own, decided that he’d try a solo walking approach. After we had determined a pick-up point in two hours, I wished him good luck and left him on the plateau. My own course took me to a narrow high-terrace of the Gart Aouirtefou, which we had not searched yet.
When I couldn’t make out Andi’s silhouette at the horizon any more I began to feel a little uneasy. I was not so sure, whether he had understood the importance to stick to our appointment by all means and not to wander off in some direction hoping that somehow I’d find him anyway. Later he told me that he was fully aware of the riskiness of his endeavour and that he had the same doubts vice versa. In the moment I had disappeared in the distance, his situation had awkwardly reminded him ‘of a lone survivor drifting in the ocean who just sees the only rescue boat float away into the wrong direction.’
When I returned in the late afternoon I was glad to see him still walking the selected grid. I was even happier when, at the sight of the car, Andi started to wave and to jump, which, unless he hadn’t suffered a sun stroke, was an unmistakeable sign that he had finally found himself a meteorite. While all finds of the team, minus the mandatory deposit samples, were shared equally between the car crews, it was still a matter of personal importance to us whether a meteorite was found in a combined team effort or whether one actually had the fortune to make a find oneself. Andi had indeed found a space rock. And not only one, but an area littered with fragments.
After receiving my due congratulations he told me, that half an hour after I had left, Thomas and Rainer had passed by in their rather quick regular search speed. When he had reached the tracks they had just left, he had spotted a meteorite right next to them, then another and yet again another. He had left the stones in their in situ positions for me to see and indeed, Thomas had passed the closest fragments only by a meter without spotting them. This, however, was understandable. Rainer, who had manned the co-driver seat, would have had to look straight into the direction of the sun to see them and from such an angle, it was hard if not impossible to tell the difference between the dark side of a local rock and a meteorite.
Andi also pointed out numerous shards of Neolithic pottery and ostrich eggs between the meteorite fragments. Both pointed to the fact that the shallow drainage channel at some point intersected a prehistoric settlement. It was due to the very low angle of inclination, probably only a few centimeters per mile, that the meteorite fragments together with the artefacts had not been washed down into the valley. After taking down the coordinates and updating the find logbook we searched the wider surroundings on foot. This lead to several more meteorite finds. The last piece that I picked up was a nice 400g fragment, badly weathered, but still displaying nice regmaglypts on one surface, which gave a faint idea of how beautiful the atmospherically sculpted meteorite must have looked shortly after its fiery descent.
During the day I had taken elevation measurements in short intervals. My working hypothesis was that the dense concentration area we had found was situated on a watershed. If this was true, the low erosion gradient of the surface might play a crucial role and this in turn was a relevant factor for the in situ preservation and weathering gradient of meteorites exposed on the surface over geological periods.
While the present time erosion in the area was minimal and rather caused by aeolian deflation, it was obvious that hydro-abrasive activity had played a much bigger role during the more humid phases. Particularly those meteorites deposited during or prior the late Pleistocene wet phase between 40 and 20 ka ago and the Holocene wet phases between 10 – 5 ka ago were affected by a higher degree of hydrodynamic activity. Displacement of surface rocks by the drain off of heavy rains as well as mechanical abrasion during the pluvials catalyzed the chemical and mechanical weathering processes a given meteorite population was subjected to.
Perhaps hydraulic gradients were a more important factor controlling the accumulation of meteorites in certain areas over time than previously thought. Unfortunately the satellite images and the topographic charts showed the orientation of the local drainage systems only to a very limited extent. More data was necessary, and I was confident to gather the necessary evidence, if not on this trip then on the ones to follow.