What Kind of Material Should I Use for the Base?

It’s best to use compactible aggregates ranging in size from 0.25 in to 1.5 in (6 to 38 mm) with no more than 10% fines.  Use this material in the base, within block cores, and behind the block.  Irregular shaped materials also work much better than rounded materials.  Crushed stone for example is far superior than rounded river rock.  The sizes of the aggregates should also be well graded, meaning they should have a good mixture of sizes.  This is important to ensure that the aggregates can be compacted easily and within tolerances.