Rambling Rodericks travel blog

John Day Monument entrance

Poster detailing the geologic layers of this area

Called "The Cathedral"

Example of John Day Formation lahars & lava flows

Greenish lahars from John Day formation

Pinkish lahars from John Day formation

Example of the 17 layers of lava flows

One of the hiking trails out to the lahars

Oregon is such a beautiful state. There are very distinct geologic and climate zones to enjoy. The Southern Oregon coast is spectacular. The mid-state valley from north to south has wonderful farms. The east side of the Cascades is supposedly desert, but we have found it is also farm land – and that mostly cattle and hay fields with some wheat farms. But the desert also has wonderful alpine like mountains and numerous lava fields.

We stayed in a tiny town called Dayville, just over 100 miles east of Bend. From Dayville we visited the John Day Fossil Beds National Monument. Unlike many other fossil beds throughout the country, in particular in Colorado and Wyoming, this fossil area is internationally famous for its abundant mammal fossils. The time period of the layers of rocks here was after the dinosaur ages.

At the visitor center we saw fossils of seeds and nuts, perfectly formed leaves from ancient maples and sycamore and sequoia and oaks, fossils of fish and birds and many mammals. The mammal fossils included pre-horses, from the three-toed small-as-a-dog type horse to the nearly-modern type horse. There were fossils of ancient sabre toothed cats and ancient pre-modern rhinos. The museum was divided into sections representing each of the geologic layers. Each section had fossils on display as well as murals depicting the plants and animals of the era, and representing how the climate was cold or warm, wet or dry, etc.

We walked several of the trails in the monument, and drove through many miles of scenic areas. We learned that the area is primarily volcanic, though not all in terms of lava flows. In fact, some of the newer land masses, 17 distinct layers, are basaltic lava flows, which after erosion of softer covering layers left what are called lava caps, crown-like basalt structures.

The other areas are made from volcanic ash and from ash and rock flows called “lahars”. Lahars generally happen when volcanic explosions melt the glaciers through which they come to the surface. The melted glaciers cause ash and mud and stones to “flow” out over the surface of the previous land like landslides, covering it in layers. The photos of greenish and reddish layers are lahars.

The following are notes taken from a museum in Baker City (not at the National Monument) which I thought very concisely explained the various layers of the land we saw in the John Day area.

The oldest, bottom most layer (in this area of the earth) is called the Goose Rock Formation. They estimate it was formed 100 million years ago. “A conglomerate of rounded rocks and gravel form where an ancient river empties into the sea. The ocean covers much of what would become Oregon Few fossils exist; mainly shells are preserved in this turbulent environment.” We saw one good example of this area, but didn't understand its significance at that point in time, so it didn't get photographed.

The Clarno Formation was laid down over the Goose Rock about 44 million years ago (mya). “Mud flows moving through warm tropical forests trap and preserve nuts, seeds, leaves and large mammals.”

The John Day Formation was about 35 mya. “Ash deposits from the early Cascades volcanoes accumulate into cliffs tinted by their mineral content A wetter climate favors growth of redwood forests and temperate woodlands. Fossils of oreodonts, sabretoothed cats, 3-toed horses, and turtles are abundant.”

Picture Gorge Basalts were formed about 17 mya. “Quiet flows of lava from cracks and fissures build up layer by layer to form extensive plateaus of basalt. No Fossils are found here.” We read that they hae been able to count 17 different lava flows over those eons. We also read that about 66 layers in all can be counted, starting with the Goose Rock formation, ending with the last one, the Rattlesnake.

The Mascall Formation was formed about 13 mya. “A somewhat wetter and warmer climate results in open grasslands and savannas. Meanwhile volcanoes to the east spew ash clouds which blanket the ground and accumulate to form white deposits. Fossils of beardogs, horses, rhinos, camels and elephants are found here.”

The top level, the Rattlesnake Formation, was formed about 6 mya. “A molten avalanche of ash roars from fissures and covers the land. Modern horses, mastadons, bears and rhinos flourished in a climate similar to today's. Their fossils are found in stream deposits under the ash flow.”

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