Monday, December 3, 2012

Caliche and Tuff Pebbles Near Yakima

I was looking at a regraded slope east of Yakima. A bit challenging as the slope regrading work had been completed months ago. The slope had been cut initially (made steeper) and then reshaped to meet an even gradient between a new drive and the slope above.

Most of the slope consisted of silt soils. One of the first things I did was check the elevation. The elevation was higher than the level that the backwater flooding that took place during even the highest ice age floods that backed up the Yakima Valley from flood waters being held back by Wallula Gap. Hence, I could rule out Missoula flood deposits. 

The property owner reported caliche on the slope. Caliche forms as a hard pan a few feet below the surface from the leaching of calcium carbonate part way down through the soils. The carbonate precipitates forming a hard pan and it is typically light colored. Sure enough I found lots of white pebbles scattered on the slope. However, most of the pebbles were volcanic pebbles of volcanic tuff.

A few volcanic tuff pebbles

I did attempt to take a few closeups of individual tuff pebbles, but my close up zoom temporarily froze up as it was a very cold morning. The tuff pebbles were likely delivered from an eruptive event in the Cascade Range the east by the Yakima River at a time prior to down cutting by the Yakima to its present elevation a few hundred feet below this site.

I did find some broken angular plate like fragments of caliche approximately a half inch thick.

Tuff pebbles on the upper left, caliche fragment on the lower right 

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