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Mysterious little rock.


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My occupation sometimes involves my working in the location of new construction where the ground surfaces have been altered to a certain degree, thus giving me a wonderful opportunity to look for any interesting rocks that catch my attention. Several weeks ago I spotted one while on a site that had been slowly filled-in for years with trucked in materials from around the region. There had been a bowling center there since the fifties and it was recently demolished with the total removal of the old parking lot pavement.


The rock was just lying with its most reveling side exposed at the ground's surface. This little rock contains an amazing amount information about its past travels. First, a little detail about its structure. It has thin bands of gold bearing quartz running its length. It was formed very deep within the earth where super heated water carried the dissolved gold and its other minerals upwards where they collected together as the materials cooled and the quartz crystals formed.




The side seen in the image above and below is the one I saw exposed at the ground's surface. It has been shaped into a double, side by side concave surface through the process of being transported by glacial movement over a very hard sub-straight.




I have designated the various sides that have certain distinguishing features as A,B,C,D,E and F.


If you notice at location C in the image above and below, the surface has been planed very smooth. Referring to the image above; A ,B and C all have planed surfaces.




The ridge at G (below) does not appear to be parallel to B but this is an optical effect caused by the quartz banding.






The ridge is actually parallel to B so this means they were likely formed during the same time period as the rock was moved by the glacier. Both F and B show weathering after being shaped while A and C are both remarkably smooth with sharper transitional edges as compared to F and B which show worn edges. It appears they are from two different periods of glacial interaction!


That seems almost impossible to have been able to occur given the time span between glacial periods. This is more likely the result of glaciation on nearby Mt. Hood (https://en.wikipedia.org/wiki/Mount_Hood) that is 80 km to the east where glaciers have advance and retreated since the end of the last glacial period at which time this whole region was covered in Ice.


Well worn side B that matches F in weathering (below).




Side A (below) that matches C in smoothness and sharp edges.




Another interesting feature is this broken chip on side F (below)




It appears that as the rock was being moved down the mountain and the double concave surface was being shaped the rock became caught on a fracture in the sub-straight causing the rock to stop and the glacier to continue on moving above it. At some point another rock being carried by the glacier began carving out the top side opposite of F. (below)




Eventually the strain was great enough to break that chip off the rock's leading edge. The glacier then at some point retreated and allowed the rock to be weathered by rain, wind and stream flow from the melting glacier. When the glacier advanced again it was once more transported and planed on sides A and C when the rock repositioned.


Such a wonderful little rock.

Edited by arc
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That seems almost impossible to have been able to occur given the time span between glacial periods. This is more likely the result of glaciation on nearby Mt. Hood (https://en.wikipedia.org/wiki/Mount_Hood) that is 80 km to the east where glaciers have advance and retreated since the end of the last glacial period at which time this whole region was covered in Ice.


While Hood has [and had] glaciers, the ice sheets of the last ice age did not cover the Portland area and extended only into Northern Washington State.


The map at left[sic] outlines in blue the extent of the Laurentide Ice Sheet 15,000 years ago.laurentide.gif

You might consult a qualified geologist at a local university for an analysis of your rock.

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While Hood has [and had] glaciers, the ice sheets of the last ice age did not cover the Portland area and extended only into Northern Washington State.



Hello Acme, we haven't talked in quite awhile. I was not referring to the Laurentide Ice Sheet of the last glaciation period but the Cascade glaciation that occurred during the same time period. There is some good accounts of them by Porter et al. Many of these papers unfortunately are now it seems behind paywalls.



A nice study by Porter that is available for free is this paper; https://notendur.hi.is/oi/AG-326%202006%20readings/Beringia%20and%20Alaska/A.%20overview/Kaufman_DevQuatSci2004.pdf


Quaternary alpine glaciation in Alaska, the Pacific Northwest, Sierra Nevada, and Hawaii

Darrell S. Kaufman1, Stephen C. Porter2 and Alan R. Gillespie2 1 Department of Geology, Northern Arizona University, Flagstaff, AZ 86001, USA; Darrell.Kaufman@nau.edu 2 Quaternary Research Center, University of Washington, Seattle, WA 98195, USA; scporter@u.washington.edu, alan@ess.washington.edu


"During their greatest Pleistocene advance, alpine glaciers in the Washington Cascade Range and Olympic Mountains terminated as much as 70–80 km from their sources. During the last glaciation, the largest glaciers were only half as long. In the Oregon Cascades, glacier tongues terminated 10–30 km from ice fields that mantled the range crest"


And this reference; http://glaciers.research.pdx.edu/glaciers-oregon

"During the Pleistocene (~1.8 million years BP - 10,000 years BP), the Oregon Cascades may have been covered by glaciers creating a small ice cap (Porter et al., 1983)"


As I mentioned, the materials had been trucked-in from all over the region, they may have been in the range of glacial flows mentioned above but unlikely to have been planed by multiple glacial movements at widely separated time periods. Definitely not at glacial period time scales. The degree of weathering between sides A and B for example is likely just several thousand years or less.


That seems almost impossible to have been able to occur given the time span between glacial periods. This is more likely the result of glaciation on nearby Mt. Hood (https://en.wikipedia.org/wiki/Mount_Hood) that is 80 km to the east where glaciers have advance and retreated since the end of the last glacial period at which time this whole region was covered in Ice.

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I have been looking for some images that show glacial flows that results in the sub-straight having similar (mirror) abrasive wear that matches my example. The ones in the image below appear to be twice or more in size than my example, but in looking at dozens of different example images it seems that these grooves can occur at a wide range of scales.





Some of these sites are deeply channeled gorge-like areas where the walls and floor are grooved by the ice, rocks and undoubtedly a particle slurry made up of a wide range of particle sizes produced through the glacial processes that work to not only lubricate but help wear down both the moving rocks, termed glacial erratics, and the sub-straight alike. I wonder, and would suppose that these gorged-out areas began from a already existing low area that the dynamics of the ice being compressed into them accelerated the erosional processes. The continually steepening walls would move that slurry to the lowest areas and accelerate the removal of the base material.




Even what appear to us as high areas were just low and of little consequence to the much more massive ice flows.




I can almost see my little rock left stranded on that precipice (above) when the glacier retreated. Eventually wind and rain tipping it over the edge and it falling into the erratic filled valley below. Maybe that is where that big chipped off spot came from.

Edited by arc
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Hello arc,


I'm sure your area shows much evidence of past glaciation activity.

And you have some good photos.


It can, however sometimes be difficult to distinguish the action of other agents.


Water plus water borne rocks can produces channels similar to the third photograph, htough I'm not suggesting that in this case.


Another agent you might like to look up is here



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Hello studiot,


Thanks for your information, that is interesting. I think I've seen that over the years in images but I didn't know the cause.


Our area is part of the Pacific temperate rain forest of the Cascade Mt. Range which results in a substantial amount of rain derived erosion of the terrain and the tremendous volumes of biomass that these soils have produced that have combined to fill in the western lowlands. The eastern side of the Cascade Range is substantially higher and dryer than the west side and reveals much more of the basaltic base material that covers so much of this area. I would think there would be images taken over the years, just to the east of the range's crest, that would show evidence of these moving masses of ice. I have only so far found a few such images.


I really like this blog BTW, she does some really good posts.






"I followed my geological instincts to this cliff edge, where I found some great glacial striations and grooves on some lava rock (possibly the andesite of Applegate Peak, which has a K-Ar date of 258±8 ka to the south - Bacon, 2008). The grooves point approximately toward the Wineglass - a northeasterly direction."



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  • 1 year later...


I have another mysterious little rock. This one is a little more interesting than you would first think. I found it when I was around eight years old while helping my parents dig a cellar next to our old farmhouse. The house was built in 1925 amongst 14 large white oak trees, some as old as 300 years or more.

 The rock is about the size of a grapefruit, appears to be sedimentary with substantial interconnected gas pocket openings penetrating into its interior regions. When I had first extracted it from the ground and cleaned its surface, a quite noticeable sound could be made by shaking the rock. It had a considerable quantity of very small shell fragments trapped inside of its interior cavities. It took some rigorous shaking to move some of the pieces to the outside. Over the years I would shake some out when showing someone my little mystery rock but I unfortunately neglected to save the pieces so now there is very little of it left inside compared to when I first found it.  




If I recall correctly the specimen was buried about 150cm in depth, encased within hard sandy clay with various sized rounded pebbles and cobbles. Most pebbles and cobbles around here appear to be mostly basaltic rather than the sedimentary material like that of the sample. The rocks were seldom close enough to contact each other and in fact were separated by enough distance to get only a few very small pebbles in every shovel full, with just several cobbles to every wheelbarrow load.

What makes this rock so unusual when compared to all the others encountered while digging that cellar and the tens of meters of ditches we dug in and around the house and property, was not only its unordinary roundness, the others were smooth but were still irregularly shaped, but that this rock had been in a saline environment PRIOR to its burial.  


An interesting surprise were these Barnacles, they appear to be Balanus crenatus;


“This species is mainly found in the sublittoral zone but can sometimes be found under stones or overhangs on the lower shore. It colonises pebbles, bedrock, shells and artificial structures. It is found in both calm and exposed waters and can tolerate low salinity levels and is found at depths of up to 60 metres (200 ft). It seems to favour habitats with strong currents and when overcrowding occurs, may be distorted to fit the space available. It is often found growing alongside another species of barnacle, Balanus balanus.”





The Pacific Ocean is 160km to the west. So the first possible scenario to explain this sample’s origin is that it was simply transported by early Native Americans to where I uncovered it. Would this rock have had some kind of utility? It was found at a depth that would seem to indicate appreciable age. An interesting point for conversation is the area of disposition is part of the Glacial Lake Missoula ponded valley system in Oregon and Washington states.



Chinookan peoples include several groups of indigenous people of the Pacific Northwest in the United States who speak the Chinookan languages. In the early 19th century, the Chinookan-speaking peoples resided along the Lower and Middle Columbia River (Wimahl) (″Big River″) from the river's gorge (near the present town of The Dalles, Oregon) downstream to the river's mouth, and along adjacent portions of the coasts, from Tillamook Bay of present-day Oregon in the south, north to Willapa Bay in southwest Washington. In 1805 the Lewis and Clark Expedition encountered the Chinook tribe on the lower Columbia.”


Map (above) of traditional Chinook tribal territory.

Chinookan-speaking groups include:

Clackamas or Cathlascans (″Those along the Clackamas River″, inhabited the Willamette Valley on the eastbank of the Willamette River as far as the Willamette Falls, above and below the Falls themselves on either bank, and along the Clackamas River and Sandy Rivers."

   Oregon City, established in 1829 by the Hudson's Bay Company, it lies at the East bank of the Willamette river just north of Willamette Falls and immediately to the south of where the Clackamas river joins the Willamette. The Falls along with the Clackamas rapids were major fishing sites for the Chinookun who lived along both banks of the rivers for thousands of years. Settlers arrived and displaced the natives from the newly designated Oregon City site and surrounding locations.  In 1844 it became the first U.S. city west of the Rocky Mountains to be incorporated, a rather recent and narrow span of emigrational history for human alteration of the site and surrounding area.

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In the information below the substantial despositional history of catastrophic Glacial Lake Missoula flood materials over the excavation site and surrounding areas is shown. The only way I can imagine the rock specimen could be buried so deeply is by substantial seasonal flooding due to heavy rains and snow melt from the surrounding tributaries of the Willamette River sometime after the rock was transported from the coast by native inhabitants. 





Another possibility is seen by the region's many past volcanic eruptions that were illustrated so well by the Mt. St. Helen's eruption and following lahar that overwhelmed the massive Columbia River with debris and graphically shows to what degree a lahar could impact the area's river channels. 

 It would seem reasonable that extreme weather or volcanic events could have caused flooding that redistributed the Missoula Flood deposits over the rock sample. The Lahar deposit shown below on the Willamette River is some 80km up river from the excavation site, a substantial temporary damming of the river could cause a tremendous flooding event.



I will need to locate the evidence for such an event that could bury the sample sometime in the last 6-7 thousand years. On the other hand, the weather related flooding may be the more reasonable solution to this mystery. 

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