Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If the soil has some mixed particle sizes and shapes it ought to hold a gentle hill just fine.
If you want a steeper hill you may have to build something like a rock wall, not have soil on the face of the steepest part, just like in nature: Hills of soil are usually softly rounded, not sheer or vertical. Where the hill is steep it is because there are rocks.
Sand and gravel flatten out really fast.