warning track

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

• noun A strip of dirt or gravel along the outfield wall, designed to warn outfielders when they are approaching the wall.

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

• noun softball, baseball The dirt or other material on the edge of a baseball or softball field that warns a player that he or she is approaching the fence, especially the portion in the outfield.