While looking for sites of relics from the South African war we came across a few of these rocks, they really intregued us, not knowing what they were we consulted Dr Sux Zietsman (retired geologist) and got the following reply ….”these rocks are shales from the Timeball Hill Formation of the Transvaal Supergroup. Those “ridges” are indeed mudcracks filled with  more resistant sediments. These more resistant crack- fillings are more resistant to weathering (contains more quartz) and subsequent weathering and erosion have removed the softer shale while the resistant crack-fillings remained. These rocks are not of high interest to geologists and they do not regard them as “special”

True enough, they do have a certain curiosity and aesthetic value, but that is all. Bear in mind that these rocks are “soft” and do not make good construction material.”

My advice is that no time and resources be spent on these rocks as they are no rare at all. They are simply “cute”