The second point that I would like to mention is the evidence of a past climate
very different from the present dry one. From Jabal Kudd, near Darb, Mr.
Philby brought back a very finely crystalline sandstone?we may call it a siltstone?
with alternating bands of finer and coarser material in layers of regular
thickness, the rhythm being repeated many times in one small sample. It
seems a fair inference that the alternations are produced by alternating wet and
dry seasons, and that the siltstone may have been laid down on the floor of lakes
by glacial rivers, which were able to bring relatively coarse material in times of
summer flood and at other times only fine silt. Such rocks are well known in
various parts of the world and many of them are thought to have been formed
on the floors of glacial lakes. It perhaps seems strange to suggest that there was
once a glacial period in Arabia, but it is a possibility. A monsoonal climate,
with well-marked wet and dry seasons, might produce such a rock.