These fossils of palm-like leaves are in the Chuckanut Formation, along Chuckanut Drive south of Bellingham. The Chuckanut Formation is important for several reasons.

The Chuckanut Formation is an overlap formation, not an accreted terrane. It accumulated as layers of sediment on top of several accreted terranes. The fact that the Chuckanut Formation overlaps several terranes shows that those terranes were already next to each other and accreted to North America by the time the Chuckanut started forming.

The Chuckanut Formation was deposited during the Eocene epoch, starting approximately 55 million years ago. This provides some age constraints on other important geological features in the area. The terranes that the Chuckanut Formation overlaps must have accreted to North America prior to 55 Ma.

The eastern part of the Chuckanut Terrane is in the North Cascades, along the Straight Creek Fault. Details of fault and fold structures in the rocks suggest that the Straight Creek Fault was active even as the sediments of the Chuckanut Formation were being deposited. This adds positive evidence of Eocene activity on the Straight Creek Fault, the major strike-slip fault that defines the western boundary of the North Cascades Crystalline Core.

The folds and faults in the Chuckanut Formation indicate that during and after the time it accumulated as layers of sediment and was buried and turned into rock, the crust in the area was subjected to strike-slip faulting and compression.

The nature of the sediments, sedimentary structures, and fossils in the Chuckanut Formation reveal what the climate and geological environment were like at the time the sediments were being deposited.

Most of the sandstones, siltstones, shales, conglomerates, and coal layers of the Chuckanut Formation were deposited in the channels and heavily vegetated floodplains of meandering rivers, during a time when the climate was significantly warmer than it is now. (Coal in the Chuckanut Formation was once mined beneath the city of Bellingham. Some of the old, abandoned mine shafts remain beneath the city.)

The mineral grains in the abundant sandstones of the Chuckanut Formation are mainly feldspars and quartz, along with some biotite, muscovite, hornblende and other minerals. This makes the sandstone in the Chuckanut Formation an arkose, which indicates that the source of the sediments was an uplifted region of granitic rock.

Tilted Beds

This picture shows that the beds of sandstone, siltstone, and coal in the Chuckanut Formation are steeply tilted in the vicinity of the fossils shown in the picture above. The compression of the crust that caused the tilting and folding of the Chuckanut Formation occurred after the sediments had been deposited and turned into rock. The compression may be due to the accretion of the terranes of the Olympic Mountains to the southwest, which began by the end of the Eocene epoch. Select the image to see a larger view. Use your browser's back button to return to this page.

View of the San Juan Islands

This picture looks west from Chuckanut Drive to the San Juan Islands, which consist mainly of terranes that accreted to North America in Middle Cretaceous to Late Cretaceous time. The Chuckanut Formation overlaps the eastern side of some of these terranes, just as it overlaps some of the accreted terranes of the western North Cascades. Select the image to see a larger view. Use your browser's back button to return to this page.

These relationships show that San Juan Island terranes and the western North Cascade terranes may have accreted during the same time, prior to the Eocene epoch when the Chuckanut sediments started to be deposited.

Fossil Tree Trunk

This photo shows the fossil trunk of an extinct tree in the Chuckanut Formation, along Chuckanut Drive south of Bellingham. The roots alongside the fossil are from modern, living plants. Select the image to see a larger view. Use your browser's back button to return to this page.

Leaf Fossils

This image shows that along with palm-like trees, the Chuckanut Formation also contains other types of fossil leaves. Studies of the plant fossils in the Chuckanut Formation suggest that the climate was several degrees warmer when it was deposited. The winters in particular were warmer than they are in the area now. The Eocene epoch was one of the last intervals of warm global climate before the gradual cooling of the Earth that lasted millions of years and culminated in the Pleistocene ice ages, which began approximately two million years ago. Glacial till, from the Vashon Lobe of the Cordilleran Ice Sheet, overlies the Chuckanut Formation. There is a gap in time of more than 35 million years between the Chuckanut Formation and the Vashon Till. Select the image to see a larger view. Use your browser's back button to return to this page.

Glossary terms that appear on this page: overlap formation; sandstone; siltstone; shale; conglomerate; coal; feldspar; quartz; muscovite; biotite; hornblende; arkose; till; glacier; ice sheet