I want to thank everyone for his or her input on this subject. Theory is ok but I’m a show me kind of person.

Experiment #2 glass on 26.5 degree ramp.

Result = glass does slide very nicely no lubricant required.

It appears concrete was the failure of experiment #1

I feel reasonably confident that polished granite plugs (could) slide by gravity to the bottom of the passage.

Experiment # 3 acquire a granite block and a limestone slab.

Thanks again.

RLH