 The dry bowl that forms the depression is marked by a series of ephemeral lakes, many of which were last filled during wetter periods of the Holocene.Diatoms from these fresh water lakes, once part of Mega-Lake Chad, now make up the surface of the depression and are the source material for the dust, which, carried across the Atlantic Ocean, is an important source of nutrient minerals for the Amazon rainforest.As the Sahara dried out over the last few thousand years, Mega-Lake Chad receded to the current position of Lake Chad in the south-west corner of Chad.