During a time when much of what is now inland Australia was below sea level , the sandstone was then covered by a layer of marine sedimentary rock shortly afterward , which formed a confining layer , thus trapping water in the sandstone aquifer .
During a time when much of what is now inland Australia was below sea level , the sandstone was then covered by a layer of marine sedimentary rock shortly afterwards , which formed a confining layer - thus trapping water in the sandstone aquifer .