Rattlesnake Formation

The Rattlesnake Formation is the mesa-like cap rock that unconformably overlies the Mascall Formation and can be seen above the hills here in the John Day River Valley. The rim rock was formed during a devastating eruptive event 7 million years ago (Mya). A proto-Cascade Range volcano (the size of Mount Hood) near Burns erupted, spewing hot ash, cinders, and other debris that scattered over 13,000 square miles of eastern Oregon. The formation is named for the type locality for the formation, which is found along Rattlesnake Creek, located within the monument.

Can you spot the paleontologists in this photo?

 

The paleontologists are in the top left. Looking at the them should give you an idea of the scale of the RAFT.

Hint: They are very small in comparison to the Tuff towering over them.

 

The lowest layer of the Rattlesnake Formation, a silt stone, contacts the upper portion of the Mascall formation.

NPS Photo

How Tough is a Tuff?

The Rattlesnake Ash Flow Tuff (RAFT) is a rock formation called a welded tuff. Tuffs are formed when hot ash is ejected from volcanic vents, lain down over a landscape, and consolidated to create a characteristic grainy textured rock with intermixed volcanoclastic particles. This eruption scattered ash that created tuffs ranging from 30 to 100 feet thick.

Below this tuff layer there is a conglomerate rock. This is a sedimentary rock that is composed of a sand grained matrix with imbricated river cobbles, suggesting that before the horrific RAFT event, 7 Mya, water coursed through the area. Meandering rivers cut into the earth, reshaping the landscape as the climate and water levels continued to change. The conglomerate layer overlies the Mascall Formation in a sequence of siltstones, claystones, and sandstones with varying degrees of lithification (consolidation and compaction of rock).

After the Rattlesnake Ash Flow Tuff

Water once again took hold in the area and created a fanglomerate (conglomerates that form an alluvial fan) layer on the rim rock. In some areas this layer is almost 65 feet thick, while in others it has pinched out. The landscape long ago would have looked similar to today, but thanks to erosional processes like water and wind the topography of the area is always slowly changing.

 

Too Tuff to Preserve Fossils?

Fossils from the Rattlesnake Formation are generally found bellow the tuffaceous portions of the unit, since the tuff likely would have been too hot to preserve any bone or trace material from animals. However, teeth from the earliest record of living beavers (Castor) in North America were found in the Rattlesnake Formation and are between 7 and 7.3 million years old. The Rattlesnake Paleontology page will tell you more about the about the dominant mammals and their habitats.