Looking down deep

In Petersham they have been building apartment blocks. The RSL which owned quite a bit of land used for carparking around its premises did a deal with a developer and the Council. The last stage is the demolition and redevelopment of the old RSL building. The club has relocated into the base of one of the new buildings.
The concrete of the ugly old building has been jackhammered into pieces and carted away and now they are digging down. I stop and look through a hole in an access gate now high above the excavation site. The ground down there is grey. What is it? Shale? That’s what it looks like. I can see they are breaking through strata. They are digging down into ancient times, through the millenia. Crush, smash, scoop. I wonder when it was that this ‘ground’ last saw the light of day?