Geology and Soils

 Geology

At a deep level, Vandevert Ranch lies at the western edge of the High Lava Plains Province of Oregon.  This volcanic zone dates back at least 5 million years and extends east beyond Burns.  The Cascade range to the immediate west of the ranch began rising about 1.7 million years ago.

In more local space and time, the ranch is near the northern end of what is known to geologists as the La Pine Basin.  The basin lies between the Cascade Mountain Range on the west and Newberry Crater on the east and encompasses the Spring River, the Fall River, and portions of the Deschutes and Little Deschutes Rivers.  The basin began dropping between the two mountain masses between 600,000 and one million years ago.  The basin’s formation was concurrent with volcanism and deposition by ancient glaciers. Periods of active volcanism and fluvial (river) and lacustrine (lake) deposition created a complex sequence of sediments hundreds of feet deep.  Newberry lavas backed up against Cascade Range lava flows starting about 700,000 years ago, blocking the channel of the Deschutes River and creating a lake and marsh environment over much of the basin.  About 200,000 years ago glaciers on the Cascade Range and Newberry Volcano changed the lake and marsh to a river environment   These river deposits created a 3–5 ft-thick layer throughout the entire basin.

A late incidence of the lake phenomenon occurred about 5050 BC when lava from Newberry blocked the Deschutes River at Benham Falls.  The resulting Benham Lake covered much of the ranch until about 1,900 years ago.

There are two lava outcrops near the ranch main gate and there is basalt (i.e. lava), either fractured or solid, from eight to twenty feet below the ground on all the lots on Schoolhouse Road where data is available.  It would seem likely that this lava came from Newberry Volcano or one of its many parasitic vents.

Well logs are notoriously unreliable but, interestingly, the log for the deep well near the Schoolhouse indicates levels of rock (obviously from volcanic activity) interlaced with levels of sand, gravel, boulders and clay that must have been deposited by glaciers, lakes, and rivers.  The deep well by the barn (closer to the middle of the Deschutes/Little Deschutes valley) shows no lava all the way down to 106 feet. 

Material at Depths Drilled – From Ranch Well Logs

 For more information see "Geological Cross-sections Nearby" in the appendices and also the 2007 USGS Scientific Report, Evaluation of Approaches for Managing Nitrate Loading from On-Site Wastewater Systems near La Pine, Oregon available on the web.  The La Pine Study Area is a roughly north-south rectangle with its northern boundary just north of Sunriver and south of Benham Falls and Lava Butte.

Soils from the Top Down

The top soil layer of organic matter is less than an inch thick.  It is less than seven thousand years old and may be much younger.  With some variations, the next approximately two to five feet of soil is volcanic ash (very small bits of rock and glass, similar to sand) from the eruption of Mt. Mazama (Crater Lake) about 7,700 years ago (5650 BC).  Thin layers of Mazama ash exist as far away as Saskatchewan. 

Soils are classified not only by their overall composition, but on the composition and color of various layers within the soil, the slope of the soil surface, and the typical water content of the soil.  Except for the riparian area, all the soils on the ranch are well suited to woodlands and are characterized by low fertility, high water tables, and susceptibility to compaction.    

A soils report done in 2006 to explore the ramifications of re-excavating the pond advised as follows:  “The dominate soil type to be expected from the excavation is judged to be a dark colored silty sand.  Seams of fine gravel and more coarse sand along with the occasional seam of finer grained soil will also be included in the mix.”  A dozen or more test pits dug in the meadow west of the pond in 2006 found occasional thin layers of clay or silty soil similar to clay.  A layer of brown/yellow soil about ten feet deep appeared to be weak rock when saturated with water but disintegrated into sand when dry. 

For more information see the Oregon Soils Map, the Ranch Soils Map, and the Geotechnical Report for the Pond Area in the appendices to this Stewardship Plan.  Also see the Soil Survey of Upper Deschutes River Area, Oregon from the National Resources Conservation Service (available on the web).  Soils reports for selected individual lots can be found in the Planning area of the Owners Only section of this web site (password protected). 

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