After they passed over the Jordan, Joshua turns toward mount Ebal and builds an altar there.
And it shall be so soon as ye are gone over the Jordan, that ye shall set up these stones, which I command you this day, on mount ‘Ebal; and thou shalt cover them with plaster. And thou shalt build there an altar unto the Lord thy God, an altar of stones.... (Deuteronomy 27:4-5 [Leeser])
What was the role of an altar in those times? How did it look?