sand crack

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A fissure in the side of a horse's hoof, often causing lameness.

from The Century Dictionary.

• noun A fissure or crack in the hoof of a horse, extending from the coronet downward toward the sole.
• noun A crack which forms in a molded brick prior to burning, due to imperfect mixing.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A vertical fissure in the wall of a horse's hoof; a quarter crack.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a fissure in the wall of a horse's hoof often causing lameness