stands of dead trees at other places along the coast in both Oregon and Washington. He wondered, “What

could have killed so many trees over so wide an area?”

Reflecting on his knowledge of earthquakes, crustal plate boundaries, and subsidence along coastlines, the geologist searched for possible explanations. “Did the trees die at the same time?” “Was their death related to nearby volcanic activity or some kind of biological blight?” “Given their coastal location, was there some relationship between the salt water and the destruction of the forests?”

He pursued his first question by dating the outer rings of the trees using carbon 14 radiometric methods. He found that they all had died about 300 years ago. As for the cause of the trees’ death, his mapping indicated no evidence for widespread volcanic deposits in the areas of dead forests. Furthermore, the trees were not burned, nor did careful examination indicate any evidence of insect infestation.

The geologist began thinking about the possible role of salt water in killing the trees. He recalled that a large section of the Alaskan coast dropped below sea level in 1964 when the tectonic plate that underlies much of the Pacific Ocean plunged beneath the North American tectonic plate that Alaska sits on as the result of a major “subduction zone earthquake.” Many square miles of coastal forests in Alaska died when the coastline dropped and they were submerged in salt water following the earthquake. He knew that a similar subduction zone lies beneath the Washington and Oregon coast and gives rise to the volcanoes of the Cascade mountains. He wondered whether the trees in Washington and Oregon might have been drowned by sea water when a large section of the coast subsided during an earthquake 300 years ago.

To check this explanation, he collected more data. He examined the sediments in the area. Well-preserved sections of sediment exposed in the banks of streams inland from the stands of dead trees showed a clean