The area that Bertha is digging in right now is glacial till, so it wouldn't be even a little surprising if it ran into a big glacial erratic. It should be able

The area that Bertha is digging in right now is glacial till, so it wouldn't be even a little surprising if it ran into a big glacial erratic. It should be able to chew through those even so, but if it ends up loose and just bounces around in front of the cutting head then it would be a problem, which is probably what's happening.