I'm going to go out on a limb and say that they aren't rocks. They are pieces of laterite.
See
http://en.wikipedia.org/wiki/Laterite and
http://www.britannica.com/EBchecked/...31578/laterite
Don't worry about the stipulation that they only form under hot, wet conditions. That is a common misconception which I'm sorry persists even in 'respectable' sources. They form quicker under these conditions but will form almost anywhere given enough time. There is certainly heaps of it around here.
They commonly form under sand dunes as the B horizon of a dune podzol (excuse me using old soil terminology but I've forgotten the new term). There were sand dunes across northern Tas during the last glacial maximum. The A horizon, being unconsolidated, has likely eroded away leaving bit of laterite near the surface.