Geologic time

Apr 21, 2011
Lahar. Colluvium. Tuff. This is the language of geology. Listen closely and it will tell you the history of this place--long before we came to be. That is, if you can decode it!


Lahar. Colluvium. Tuff.
This is the language of geology. Listen closely and it will tell you the history of this place--long before we came to be. That is, if you can decode it! 

Yesterday, on a walk with Forest Service geologist Bart Wills, a group of Land Trust volunteers did just that. We followed Bart down the road at Rimrock Ranch into the Whychus Creek canyon and were carried back millions of years to get a glimpse of how Rimrock Ranch--and Central Oregon--used to be. 

Evidence of lava flows, lahars, and massive rivers tell the story of a very different landscape than we currently see. I was struck by the idea that the rounded rocks, aka cobbles found in one layer meant a river flowed this way--a river that wasn't Whychus Creek. Can you imagine that? Ancient creeks and rivers oriented in no where near the same direction as they are today?

Though the millions of years that each layer represented still swim around in my brain, the words that described some of the layers stayed with me. My favorite:

KM_RR_geology tour2011
KM_RR_geology tour2011
KM_RR_geology tour2011

 Lahar. A lahar is a type of mudflow or debris flow composed of pyroclastic material, rocky debris, and water. These mudflows are found higher up on the road down into the canyon suggesting younger more recent flows. Lahars always seem impressive to me because, in the world of volcanic eruptions, these flows move quickly and destroy everything in their path. Imagine that destructive event sweeping over what is now Rimrock Ranch?

Colluvium. Further down the road we found colluvium. You mean alluvium? Nope. A word that describes the sediment that has been deposited or built up at the bottom of a low-grade slope or against a barrier on that slope. In part what identifies this deposit is the angular rocks that appear and are poorly sorted--they aren't with similar size rocks. Like an alluvial fan created at the mouth of a river, a colluvium is created at the base of a slope. At Rimrock the colluvium seemed to indicate that once the cliff walls were closer in, and that they dropped large angular rocks into the mix.

Tuff. These light colored layers are consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is loose and can be easily broken apart. Welded tuff--like Smith Rock--is cemented and is hard as rock. At Rimrock we saw evidence of both. My favorite thing about tuff: rock that contains greater than 50% tuff is considered tuffaceous. Tuffaceous!


Ecology, geology, and science in general are full of jargon. But just like rocks, if you break the jargon down, you can find and even enjoy the meaning of these terms. Have at it!