The Earth Spell feat says:
As long as you are standing on stone or unworked earth (including normal soil). You can use the Heighten Spell fear to added effect. If you cast a spell using a spell slot one level higher than the spell's actual level, the spell is treated as a spell of two levels higher and your effective caster level is increased by one. If you use a spell slot two levels higher, the spell is treated as three levels higher and your effective caster level is increased by two, and so on. You cannot gain the benefit of this feat when casting a spell with the air, fire, or water descriptor.
I see two different interpretations here:
This specific wording supersedes the Heighten Spell feat. I cast an 8th level spell using a 9th level spell slot, giving me a +1 CL and an effective 10th level spell.
This specific spell slot illustrates how the Heighten Spell feat works, and then changes one part of it. I cast an 8th level spell using a 9th level spell slot, giving me a +1 CL and an effective 9th level spell, because Heighten Spell can only heighten a spell to 9th level.
I'm fairly certain that interpretation #2 is RAI, but what is the RAW on this?