For the higher-level excavations, there are more layers of dirt, and when you place the augur over any of these squares, it highlights red to indicate you can't use the augur there. What you then need to do is use the brush to remove layers until the augur is no longer blocked. In other words, this is a mechanism to make the excavation tougher for higher levels.All the purple leads have 4-6 levels of dirt in the excavation sites and the augur doesn't work on them. Is there a way to upgrade it? I didn't see any skill for that. The brush supposedly upgrades at rank 7 but nothing is said about the augur.