FORWARD TOWNSHIP, Pa. -- Fly ash and water swept through part of Forward Township on Wednesday, one day after a muddy hillside gave way and covered the streets with tons of thick, black muck.

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Video: See The Spill, DEP Response

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Fearing that the problem could worsen, township officials told Rokosky Ridge Road residents late Wednesday afternoon to immediately evacuate their homes.

State environmental officials believe fly ash, the by-product of burning coal, was dumped at the top of the hill more than 50 years ago.

Now, the hillside resembles a dark crater, measuring about 300 feet wide and 100 feet deep.

Another problem is that the hill covered a creek that flows into the Monongahela River.

Environmental officials said the ash buildup served as a dam of sorts, blocking the flow from two feeder streams. When it gave way, the water and muck went with it.

Crews are trying to clear it out, but are worried if it backs up, more of the hill could come down.

Department of Environmental Protection officials have not said whether they believe a health threat exists, but one DEP specialist told Channel 4 Action News reporter Janelle Hall that current regulations "would not allow disposal of fly ash next to a stream, let alone right over a hillside on a highway."