New Elementalist Spell: Skyland

As I've been working on the revised Theorems & Thaumaturgy, I've needed to fill in a few gaps here and there in the spell lists. Some new spells have thus crept their way into the revised book :) Here's one, for the elementalist. (You may notice the extended "stat block" -- all of the spells in the revised edition have these extra details, as appropriate.)

Skyland
Level: 8
Duration: Permanent
Range: 0
Casting Time: 1 hour
Elements: Air, Earth
Schools: Enchantment

The ground beneath the elementalist's feet rumbles, grinds, and, at the end of the hour-long ritual, cracks and rends as a rough hemisphere of earth and stone separates from the surrounding matter and ascends into the sky. The size of the hemisphere depends on the elementalist's experience level: a 40' diameter area at 15th level, increasing in overall breadth by 20' per level above 15th  (a 20th level elementalist may thus levitate a 140' diameter hemisphere). (It may, at the Labyrinth Lord's discretion, be possible to further increase the size of the skyland by augmenting the ritual with sacrifices of precious substances or magical items of great value.)

Once untethered, the rocky hemisphere floats at a height of 150' above the surface of the earth. Casting this spell underground has no effect and, even when aboveground, the presence of any form of building in the affected area prevents the enchantment from taking hold. Buildings may, however, be constructed freely upon the surface of the hemisphere once it is afloat.

An elementalist of 7th level or greater who stands atop a skyland may command it to move through the air. This requires the summoning of an air elemental of 8 or more Hit Dice, which is magically bound into the floating hemisphere for a period of 24 hours. During this time, by concentrating, the summoner may cause the skyland to move in any direction (including vertically) at a rate of 110' per turn (3 miles per day). It requires one full turn of concentration to cause the skyland to stop or to change course. After 24 hours have passed, the bound air elemental escapes and the skyland stops moving.

While moving through the air, the floating hemisphere emits an ominous droning sound which may be heard from half a mile distant.