A thin scum of ice covered the pool beside which the column had camped.

Thermophilic algae streaked its waters, the pink ones forming a thin scum on top and nearly screening the meshed tangles of the larger, thicker blue-grey plants a short distance underneath.

He had to break the thin scum of ice on top of the stream so the horse could get a drink.

The dust floated on the surface of the water-holes in a thin scum, and it turned to mud in their nostrils and grated between their teeth.

A black crust had formed along the top, a thin scum of solid metal that seemed to breathe as it cooled.

They seemed to race clear to the horizon, where a thin scum of day's last light lay like self-luminous chemical waste, and rebound in a heartbeat.

By eight the sun was hot in the back garden, the sky a still, absorbent blue, and Josh's paddling-pool had a thin scum of dead insects and grass floating on it.

He slipped only once, his left shoe breaking through a thin scum of ice and dipping into cold and somehow slimy standing water.

I noticed too there was a thin sulphurous scum drifting upon its coiling water.

A lower population means a lower level of economic activity, which spells poverty for all but a thin scum at the top.