DC,

many stones have a weight per volume (specific gravity) of about 2.0 - 3.0 kg/dm^3. There is about 28 dm^3 in a cu ft, and a pound is .454 kg.

So a cu. ft. of granite (at 2.75) is 171 lbs concrete / mortar is about the same (160-180 lbs/cu ft depending on the aggregate)

7 ton is 14,000 lbs - 82 cu ft or 3.05 yards o f stone/concrete.

So a rectangle 23'9" wide by an average of 30" high would be about 60 cu ft / 5.12 tons if it was 1 ft thick on average.

A 7 ton structure would be just under 1.4 ft thick on average.

Hope this helps

Patrick