This rapid burial kept the forest's plant life from decomposing and allowed it to be preserved.
Geologist John Nelson, also with ISGS, found the fossils in 2004 when he was visiting the mine and noticed plant imprints in its shale-covered ceiling.
Elrick said, "Imagine an artist's canvas that's covered in gray flat paint—that's what gray shale kind of looks like.
"The plant fossils stand out in that grayness as black impressions, and they look just like pressed leaves in a book.
"As [workers] continued to mine, they exposed more and more fossils," he added.
Nelson contacted Elrick and other colleagues—including paleobotanists William DiMichele of the National Museum of Natural History and Howard Falcon-Lang, of U.K.'s University of Bristol—to invite them to see the mine.
"It stared you right in the face—you couldn't avoid it," Elrick said of the mine's fossil record.
"There were a huge number of plant fossils and not just little fern impressions but monster tree trunks.
"Imagine going into the Oregon forest today, flattening everything down, covering it with sediment, and then being able to examine the preserved forest from below.
"It's very rare that you get to see a snapshot in time over such a large area," he said.
(Read related story: "Newfound Fossils Reveal Secrets of World's Oldest Forest" [April 18, 2007].)
The discovery's sheer size provides an unprecedented view of ancient forest life, said Kirk Johnson, chief curator at the Denver Museum of Nature and Science, who was not involved in the study.
"The key thing here is that [the study team] looked at a huge area," Johnson said. "Fossil forests don't typically preserve that much area at all.
"You might get a few trunks or a small area, but by taking advantage of extensive coal mines, these guys were able to look at one forest over a thousand hectares of area."
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