Following the famous Cretaceous-ending asteroid impact and subsequent dying off of dinosaurs and many large reptiles 65.5 million years ago, our area was a large, shallow, warm sea dotted with volcanic islands and filled with coral and oyster reefs. Along the east side of the sea, swamps grew on low slopes near the water near present-day Centralia and Chehalis. Plants grew in these swamps that later formed layers of coal. In fossil-speak these are called coal swamps. This sea persisted for 50 million years, to around 20 million years ago, in what geologists call the early Miocene.
Many marine fossils are found in rocks from this period, including snails, clams, corals, crinoids, brachiopods, barnacles, sharks teeth, fish, whales, seals and turtles. Burrowing shrimp from 45 million years ago were found in marine sediments; similar shrimp species still live in Willapa Bay today.
These geologic periods had wet, warm climates and considerable volcanic activity due to a nearby subduction zone, where one of earths enormous crustal plates collides with another, a phenomenon that continues to result in earthquakes and powerful tsunamis. Water-washed ash mixed with marine silts and sands makes a very good fossil-preserving combination.
A distinctive round rock called a concretion often forms in marine sediments. As fossilization proceeds, sediments cement together to make round rocks, with the fossil at the center. Concretions form easily with small shells and crustaceans, such as shrimp, barnacles and crabs.
An outstanding sedimentary rock formation, the Lincoln Creek Formation, is from this period. The Lincoln Creek Formation is 2,000 to 9,000 feet thick, composed of tuffaceous (ashy) siltstone to fine-grained sandstone, and formed 37 million years ago. It was originally described in scientific research at a site on Lincoln Creek, off the Chehalis River in the Grays River Basin in Lewis County. It covers about 1,500 square miles in Southwest Washington, including areas of Pacific and Wahkiakum counties. This formation has a good exposure on the surface along the Willapa River east of Raymond.
Mollusks and crustaceans are common in the Lincoln Creek Formation, as are microscopic marine plankton. Crabs are particularly common. Karla Nelson at Time Enough Books in Ilwaco and her family often camped on Lincoln Creek when she was a child, and they collected concretions. When opened, these concretions typically contain fossilized crabs.
Swampy shorelines persisted in lowlands along the west side of the Cascades during the Paleocene to early Miocene Period. Trees in these swamps included palms and many conifers, mallows, species in the rose family (hawthorn, spiraea, amelanchier, sorbus, prunus, rubus), and also gingko, banana, magnolia and grasses. Specimens of many plant and animal fossils from this period can be seen at the Burke Museum (www.burke museum.org) at the University of Washington in Seattle.
The most similar modern comparison to those ancient coal swamps are mangrove thickets in the modern-day tropics. In todays world, the closest comparison to the ancient tropical shallow sea is the Southeast Asian island nation of Indonesia including earthquakes, tsunamis and active volcanoes.
Kathleen Sayce, ecologist, lives on Willapa Bay north of Nahcotta and writes on a variety of topics about natural history of this area.