tail·ing /ˈteɪlɪŋ/

1. the part of a projecting stone or brick tailed or inserted in a wall.
2. tailings,
a. Building Trades . gravel, aggregate, etc., failing to pass through a given screen.
b. the residue of any product, as in mining; leavings.

1640–50; tail + -ing

Source: Dictionary.com.

Ash had begun to rain down on them, unceasing now, each flake cold as snow, yet this was a sludge that did not melt, instead churning into the mud until it seemed they walked through a field of slag and tailings.

Source: Toll the Hounds by Steven Erikson.