Away from the coast the center of Arabia is a vast area of gravel plain partially covered in sand dunes. The main sand desert in the eastern part of the peninsula, the Rub al-Khali ( the Empty Quarter), has a much smaller southern arm known as the Ramlat as-Sabatayn. It is 100km wide at its western end near the Yemen foothills, tapering to about 15km as it enters Wadi Hadramout.
It consists of chains of dunes or uruq (urq) which are roughly parallel, with a general east west orientation, separated by narrow flats of gravel, silt or gypsum locally called shuquq (shuq).
Some of the dune ridges can be as much as 50m high and they are often several kilometers long. Interdunal lakes once formed at the end of wadis flowing into the centre of the desert.
This has given rise to the strange phenomenon, visible when crossing the desert, of what appears to be seashells in the desert sand. They are not in fact seashells but part of a Quaternary molluscan fauna which inhabited the area. They are a combination of terrestrial and aquatic snail shells (gastropods).
The internal desert lakes dried up eventually with the onset of the present climatic regime between 4000 and 3000 BC, leaving their unfortunate residents somewhat high and dry.