It was probably under this climate that the deep weathered, iron-rich profiles that characterise much of Australia were formed. Aridity only seems to have set in after Australia reached its present latitude, and the northern part was probably never arid. Today a large part of Australia is arid or semi-arid. Sand dunes are mostly longitudinal, following the dominant wind directions of a high pressure cell. The dunes are mostly fixed now.
Salt lakes occur in many low positions, in places following lines of ancient drainage.
They are often associated with lunettes, dunes formed on the downwind side of lakes. Many important finds of Aboriginal pre-history have been made in lunettes. Despite the prevalence of arid conditions today, real aridity seems to be geologically young, with no dunes or salt lakes older than a million years. The past few million years were notable for the Quaternary ice age. In Tasmania there is evidence of three different glaciations - the last glaciation, one sometime in the Quaternary, and one in the Tertiary.