Boulder Mystery

[Acme]

So there is this boulder about 7 ft. tall, 8 ft. wide, and 13 ft. long sitting in a depression of a wet prairie remnant 5 miles from the Columbia River.

 

 

The constituent rock is porphyritic and I'm thinking andesite.

 

 

1/2 mile NE of the boulder is a cinder cone dated at 575+-7KA and classified by geological study as "Olivine phyric basaltic andesite erupted from cinder cone... Light-gray, microvesicular, generally platy lava flow, consists of olivine phenocrysts (2-4 percent; 0.5 to 3 mm across; contains inclusions of chromian spinel; rims variably replaced by iddingsite) in a fine-grained trachytic groundmass of plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxide; locally contains quartzite pebbles and small, dark, fine-grained clots that may be sedimentary xenoliths, both presumably derived from underlying gravels." (I have looked at samples of rock from the cinder cone stump and they do not resemble the boulder rock)

 

The material underlying the boulder is described as "Lake deposits (Holocene and Pleistocene) Unconsolidated black to gray silt, mud, and organic debris underlying wide flat valley; grade into fine-grained alluvium (Qa) and peat deposits (Qp); overlie gravel probably deposited by cataclysmic floods (Qfg) and hyaloclastic sedimentary rocks (Ttfh); lower part may include Missoula-flood slack-water deposits (Qfs). Sparse well logs indicate deposit is less than 5 m thick."

 

I am mystified as to how such a large and old boulder is sitting atop such fine, deep, and young sediments. ?

Edited January 22, 2017 by Acme

[studiot]

Hi Acme, the Columbia river huh?

 

Canada to Frisco?

 

You don't give any clue as to whereabouts, but the CR is an very important north american river and could easily move such a boulder many miles when in spate.

 

Do you know where the parent rock is located?

[Acme]

Hi Acme, the Columbia river huh?

 

Canada to Frisco?

Hi Studiot. Erhm, Canada to Portland Oregon, thence a jog and wiggle to Pacifica Oceana.

 

You don't give any clue as to whereabouts, but the CR is an very important north american river and could easily move such a boulder many miles when in spate.

The exact location is on a preserve so I'm not inclined to give it as entry is restricted. (Yes, I have permission. ) However, the area is in Washington state more-or-less across the Columbia from Portland.

 

Do you know where the parent rock is located?

I do not, other than likely the Cascade mountain range. My geology chemistry is rather rusty, and it wasn't all that shiny when I was studying. I'm hoping Ophiolite happens by and shares his wisdom.

 

I do have something of an hypothesis, but I didn't give it as I didn't recall the terms. Anyway, I went looking and that terminology would be granular convection. Even so, this is an awful big rock and I'm not confident such convection could lift something this big.

 

Granular Convection and Size Separation

 

(Inverse)Graded bedding

[zapatos]

Perhaps something like the Missoula Floods.

The Missoula Floods (also known as the Spokane Floods or the Bretz Floods) refer to the cataclysmic floods that swept periodically across eastern Washington and down the Columbia River Gorge at the end of the last ice age. The glacial flood events have been researched since the 1920s. These glacial lake outburst floods were the result of periodic sudden ruptures of the ice dam on the Clark Fork River that created Glacial Lake Missoula. After each ice dam rupture, the waters of the lake would rush down the Clark Fork and the Columbia River, flooding much of eastern Washington and the Willamette Valley in western Oregon. After the rupture, the ice would reform, creating Glacial Lake Missoula again.

...

https://en.wikipedia.org/wiki/Missoula_Floods

 

Along the floodwaters path, more than 50 cubic miles of earth and rock were removed, transported, and much was deposited as new landforms. The floods built gravel bars as tall as 400 feet and moved boulders weighing many tons and deposited them high on the valley walls.

http://columbiariverimages.com/Regions/Places/missoula_floods.html

[Acme]

Perhaps something like the Missoula Floods.

https://en.wikipedia.org/wiki/Missoula_Floods

 

http://columbiariverimages.com/Regions/Places/missoula_floods.html

Well, there are Missoula flood deposit there, but they underlie the prairie. (see geologic study quote in post #1) Those flood rocks in the boulder area are at most a couple feet in diameter and rounded. As the water spread and slowed in this area, the heavier stuff dropped out first, so if this boulder was from those floods it should be buried it seems to me. And 13,000 years of sediments washing down the valley since should have buried it too. ?

 

There is a nearby manmade lake that they may have dug the boulder from, but it's not very deep. I'll have to go see if someone there knows any history on it.

[arc]

Acme, what you have found is a tephra that is probably from the Smith Creek Eruptive Period that occurred between 3.9 - 3.3 thousand years ago. As the link below testifies, these tephra materials have been found "as far away as 950 km (590 mi) from source."

 

https://volcanoes.usgs.gov/volcanoes/st_helens/st_helens_geo_hist_108.html

 

Smith Creek Eruptive Period (3.9 to 3.3 ka)

"During the Smith Creek period, two periods of highly explosive activity (3.90 to 3.85 ka and 3.5 to 3.3 ka) deposited large amounts of tephra (set "Y") and pyroclastic flows. The second period was initiated with a highly explosive eruption ("Yn") that was about four times larger than the one in 1980, making it the most voluminous eruption in Mount St. Helens' history. These tephras have been identified as far away as 950 km (590 mi) from source. During late Smith Creek time, a lava dome was extruded and huge lahars swept down the Toutle River and probably reached the Columbia River."

 

Tephra

Any type and size of rock fragment that is forcibly ejected from the volcano and travels an airborne path during an eruption (including ash, bombs, and scoria).

"As in earlier stages, Spirit Lake volcanism erupted mostly dacite, but significant amounts of basalt and andesite were also erupted. The Spirit Lake Stage is subdivided into six eruptive periods—the Smith Creek, Pine Creek, Castle Creek, Sugar Bowl, Kalama, Goat Rocks, and the Modern period of activity that began in 1980."

http://www.geo.mtu.edu/volcanoes/hazards/primer/tephra.html

"When a volcano erupts it will sometimes eject material such as rock fragments into the atmosphere. This material is known as tephra. The largest pieces of tephra (greater than 64 mm) are called blocks and bombs. Blocks and bombs are normally shot ballistically from the volcano (refer to the gas thrust zone described in the direct blast section). Because these fragments are so large they fall out near their source. Blocks and bombs as large as 8-30 tons have fallen as far away as 1 km from their source (Bryant, 1991). Small blocks and bombs have been known to travel as far away as 20-80 km (Scott, 1989)! Some of these blocks and bombs can have velocities of 75-200 m/s (Bryant, 1991). Smaller ejecta such as lapilli (2-64 mm) and ash (<2 mm) which are convected upward by the heat of the eruption will fall out farther from the volcano. Most particles greater than a millimeter in size will fall out within 30 minutes of the time they are erupted (W.I. Rose personal communication). The smallest particles which are less then .01 mm can stay in the atmosphere for two or three years after a volcanic eruption."

That is a nice find I hope you kept a souvenir er I mean sample.

Edited January 24, 2017 by arc

[Acme]

Acme, what you have found is a tephra that is probably from the Smith Creek Eruptive Period that occurred between 3.9 - 3.3 thousand years ago. As the link below testifies, these tephra materials have been found "as far away as 950 km (590 mi) from source."

... The largest pieces of tephra (greater than 64 mm) are called blocks and bombs. Blocks and bombs are normally shot ballistically from the volcano (refer to the gas thrust zone described in the direct blast section). Because these fragments are so large they fall out near their source. Blocks and bombs as large as 8-30 tons have fallen as far away as 1 km from their source (Bryant, 1991). Small blocks and bombs have been known to travel as far away as 20-80 km (Scott, 1989)! ...

 

That is a nice find I hope you kept a souvenir er I mean sample.

Yes, I collected a sample. It is pictured in the second 2 photos of my original post. I regularly report all my findings to state officials and seek their direction for dispensation of anything I collect. (My primary activity on the site is botanical in nature.)

I don't think this is tephra as it is quite hard and dense. The apparent siliceous crystals do not appear to be cemented fragments as in a breccia, rather formed in situ during cooling as in andesite. The groundmass appears basaltic to me. Here, I am unsure of the minerals' ID. I no longer have a mineral handbook, but I do have a scratch plate and may have a go at getting some streak info.

 

Ophiolite! You got your ears on? :-D

 

Anyway, using an online calculator and using a volume of 728 ft³ (7'x8'x13') & presuming it is andesite as I surmised, the boulder would weigh approximately 125,000 lbs. (62 tons) and so by your own source could not have landed in its present location as a bomb from St. Helens which is ~50 miles(30km) north.

 

Neither could the boulder have washed from St. Helens as the prairie drainage comes from the east and drainage south off of St. Helens goes into the Lewis R. drainage thence west to the Columbia.

 

I would entertain the notion that the boulder's source was Mt. Adams which is ~ 60 miles NW, but again not as a bomb. (Eruptive history of Mt. Adams)

 

The mystery of the boulder's mineralogy and source aside, I am mostly curious about how it came to be sitting on top of relatively young sediment. I have yet to check on the age of the nearby man-made lake, but as I say it is not very deep and from an engineering standpoint I would think workers would break it up if encountered rather than moving the whole shebang.

[Ophiolite]

My immediate reaction on seeing Columbia mentioned was the Missoula floods. I see zapatos had the same thought.

 

The tephra explanation from arc also seems plausible. I think you may have misunderstood the nature of tephra. The suggestion is not that the boulder was formed by the fusion of ash and larger crystals into a tuff. The idea is that a fragment of previously consolidated magma was ejected as a discrete piece.

 

As to the hand specimen, the large crystals appear to feldspar, while the ground mass looks generally basaltic, based upon the colour. How sharp are the crystal boundaries of the phenocrysts? That would give insight as to the extent of disequilibrium/equilibrium between the phenocryts and the then liquid magma.

[Acme]

My immediate reaction on seeing Columbia mentioned was the Missoula floods. I see zapatos had the same thought.

First, thanks for having a look at this.

So on the Missoula flood idea, see again what I quoted from a USGS geological survey map of the area where the boulder sits. I have bolded the pertinent part:

...

The material underlying the boulder is described as "Lake deposits (Holocene and Pleistocene) Unconsolidated black to gray silt, mud, and organic debris underlying wide flat valley; grade into fine-grained alluvium (Qa) and peat deposits (Qp); overlie gravel probably deposited by cataclysmic floods (Qfg) and hyaloclastic sedimentary rocks (Ttfh); lower part may include Missoula-flood slack-water deposits (Qfs). Sparse well logs indicate deposit is less than 5 m thick."

What I am not understanding is if the boulder is a Missoula flood artifact, how is it sitting atop finer Missoula flood material since larger material would drop out first? Is my speculating on granular convection a non-starter?

 

The tephra explanation from arc also seems plausible. I think you may have misunderstood the nature of tephra. The suggestion is not that the boulder was formed by the fusion of ash and larger crystals into a tuff. The idea is that a fragment of previously consolidated magma was ejected as a discrete piece.

Roger that. As I responded to Arc, the boulder is too large to have fallen where it is as a bomb from either St. Helens or Adams, and could not have washed where it is from St. Helens. This leaves Adams as a source, but when a water flow sufficient to carry so large a boulder -the valley carries a creek at present- slows, the boulder should drop out first and the finer material should fall out later and bury it. Oui/no?

 

As to the hand specimen, the large crystals appear to feldspar, while the ground mass looks generally basaltic, based upon the colour. How sharp are the crystal boundaries of the phenocrysts? That would give insight as to the extent of disequilibrium/equilibrium between the phenocryts and the then liquid magma.

By 'sharp' I presume you mean abrupt. If so, the boundaries are quite sharp and there does not appear to be any grading between phenocrysts and groundmass.

 

I can add that I have no idea how much boulder is below the surface. While chipping a small piece was OK, any digging would require a formal permit application and approval. As the preserve is principally founded on biota and I am not suitably qualified to conduct geological research, I doubt such activity is in the cards simply to satisfy my idle curiosity. D'oh!

 

Thanks for the input.

[zapatos]

What I am not understanding is if the boulder is a Missoula flood artifact, how is it sitting atop finer Missoula flood material since larger material would drop out first? Is my speculating on granular convection a non-starter?

What do you mean by 'drop out first'?

(Just to be clear I have no expertise on this subject and so cannot add much. I just find it interesting.)

[Acme]

What do you mean by 'drop out first'?

(Just to be clear I have no expertise on this subject and so cannot add much. I just find it interesting.)

Love the interest! So it takes a larger flow of water to move a boulder than a pebble, so when a flow sufficient to be moving a boulder slows below the threshold to move the boulder, the boulder will drop to the bottom of the channel, while pebbles will continue to be moved along. When the flow drops below the pebble threshold, the pebbles drop out while sand would keep flowing. Then sand drops out with more slowing, then silt particles still flow until water slows to stop.

 

I think I linked to this article earlier, but it gives a more rigorous explanation so worth reposting. >> Graded bedding

[zapatos]

Love the interest! So it takes a larger flow of water to move a boulder than a pebble, so when a flow sufficient to be moving a boulder slows below the threshold to move the boulder, the boulder will drop to the bottom of the channel, while pebbles will continue to be moved along. When the flow drops below the pebble threshold, the pebbles drop out while sand would keep flowing. Then sand drops out with more slowing, then silt particles still flow until water slows to stop.

 

I think I linked to this article earlier, but it gives a more rigorous explanation so worth reposting. >> Graded bedding

Thanks!

In the article you linked it mentions that "In reverse or inverse grading the bed coarsens upwards. This type of grading is relatively uncommon but is characteristic of sediments deposited by grain flow and debris flow.[1] It is also observed in eolian ripples. These deposition processes are examples of granular convection."

 

Since eolian ripples were generated by the Missoula Floods, is it possible you are looking at the remains of one of those ripples, with the coarser material (your large boulder) sitting atop finer particles?

[Acme]

Thanks!

In the article you linked it mentions that "In reverse or inverse grading the bed coarsens upwards. This type of grading is relatively uncommon but is characteristic of sediments deposited by grain flow and debris flow.[1] It is also observed in eolian ripples. These deposition processes are examples of granular convection."

 

Since eolian ripples were generated by the Missoula Floods, is it possible you are looking at the remains of one of those ripples, with the coarser material (your large boulder) sitting atop finer particles?

Mmmm...Wiki doesn't have a page on eolian ripples, but USGS does and it says this is a wind driven phenomenon. > Eolian Processes

 

However, the debris flow mentioned is more plausible. Still, the immediate underlying material is not composed of debris likely to accompany the boulder according to the survey map notes: "Lake deposits (Holocene and Pleistocene) Unconsolidated black to gray silt, mud, and organic debris underlying wide flat valley; grade into fine-grained alluvium (Qa) and peat deposits (Qp); overlie gravel probably deposited by cataclysmic floods (Qfg) and hyaloclastic sedimentary rocks.

 

The (Qfg) description is: "Gravel facies -- Unconsolidated bouldery pebble to cobble gravel ... Underlies creek valley, part of a large bar formed to the south. Poorly sorted; clast-supported; contains well-rounded to subangular clasts as large as 2.5 m diameter; some open-work gravel, but most contains matrix of basaltic to arkosic sand. Excavations reveal foreset bedding with west to northwest dips as great as 25°. Clast population dominated by Columbia River Basalt Group and Pliocene or younger basalts from the Cascade Range; commonly includes Tertiary volcanic rocks, pre-Tertiary granitic and metamorphic rocks, and quartzite. Well logs indicate that gravel grades into mixed sand and gravel.

 

Soooo, if the boulder were the tip of an iceberg so to speak, there may me 15ft more of it below the surface that is surrounded by the sedimentary lake deposits but sitting more-or-less on top of cataclysmic flood or debris flow material that delivered it. ?

 

A monolith wrapped in a mystery inside an enigma.

Edited January 25, 2017 by Acme

[zapatos]

While reading through several articles I seem to have confused the aeolian ripples with the Giant Current Ripples created during the Missoula Floods.

https://en.wikipedia.org/wiki/Giant_current_ripples

[Acme]

While reading through several articles I seem to have confused the aeolian ripples with the Giant Current Ripples created during the Missoula Floods.

https://en.wikipedia.org/wiki/Giant_current_ripples

10-4. You can't be any more confused than I. I like big rocks and I can not lie...

[Moontanman]

So there is this boulder about 7 ft. tall, 8 ft. wide, and 13 ft. long sitting in a depression of a wet prairie remnant 5 miles from the Columbia River.

 

 

The constituent rock is porphyritic and I'm thinking andesite.

 

 

1/2 mile NE of the boulder is a cinder cone dated at 575+-7KA and classified by geological study as "Olivine phyric basaltic andesite erupted from cinder cone... Light-gray, microvesicular, generally platy lava flow, consists of olivine phenocrysts (2-4 percent; 0.5 to 3 mm across; contains inclusions of chromian spinel; rims variably replaced by iddingsite) in a fine-grained trachytic groundmass of plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxide; locally contains quartzite pebbles and small, dark, fine-grained clots that may be sedimentary xenoliths, both presumably derived from underlying gravels." (I have looked at samples of rock from the cinder cone stump and they do not resemble the boulder rock)

 

The material underlying the boulder is described as "Lake deposits (Holocene and Pleistocene) Unconsolidated black to gray silt, mud, and organic debris underlying wide flat valley; grade into fine-grained alluvium (Qa) and peat deposits (Qp); overlie gravel probably deposited by cataclysmic floods (Qfg) and hyaloclastic sedimentary rocks (Ttfh); lower part may include Missoula-flood slack-water deposits (Qfs). Sparse well logs indicate deposit is less than 5 m thick."

 

I am mystified as to how such a large and old boulder is sitting atop such fine, deep, and young sediments. ?

 

 

It's a good chance a glacier dropped it.

 

https://en.wikipedia.org/wiki/Glacial_erratic

 

 

A glacial erratic is a piece of rock that differs from the size and type of rock native to the area in which it rests. "Erratics" take their name from the Latin word errare (to wander), and are carried by glacial ice, often over distances of hundreds of kilometres. Erratics can range in size from pebbles to large boulders such as Big Rock (15,000 tonnes or 17,000 short tons) in Alberta.

Geologists identify erratics by studying the rocks surrounding the position of the erratic and the composition of the erratic itself. Erratics are significant because:

[Acme]

It's a good chance a glacier dropped it.

 

https://en.wikipedia.org/wiki/Glacial_erratic

There were no glaciers here when the Missoula floods were occurring. ('Here' being in Washington more-or-less across the Columbia from Portland Oregon)

Glaciers of Washington

The mystery here is why this boulder is [apparently] sitting on top of ~15 ft. of lake deposits.

 

Since flood waters develop normal bed grading and debris flows develop inverse bed grading, perhaps one of the later floods triggered a debris flow that deposited the boulder.

 

As I said, I don't know how much of the boulder is beneath the surface and it's also a possibility it was dug up and moved when a nearby lake was constructed. I may be able to talk to someone who knows the history of the lake construction, but it is unlikely I will be allowed to do any excavating.

Addendum:

There is also the other possibility that I proposed, i.e. granular convection:

Even so, this is an awful big rock and I'm not confident such convection could lift something this big.

Granular Convection and Size Separation

Is it possible that millennia of freeze/thaw cycles could lift this size of boulder? Edited January 27, 2017 by Acme

[arc]

 

 

It's a good chance a glacier dropped it.

 

https://en.wikipedia.org/wiki/Glacial_erratic

 

 

I had a thread previously on this subject; http://www.scienceforums.net/topic/98478-mysterious-little-rock/#entry944470

 

And also mentioned this situation is not reliant on the Larentide Ice Sheet to distribute these materials during the last glacial.

 

http://www.scienceforums.net/topic/98478-mysterious-little-rock/#entry943609

"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....QuatSci2004.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"

The 70-80 km estimate is within range to derive this specimen from Mt. St. Helens.

"St. Helens which is ~50 miles(30km) north."

Edited January 28, 2017 by arc

[Acme]

I had a thread previously on this subject; http://www.scienceforums.net/topic/98478-mysterious-little-rock/#entry944470

And also mentioned this situation is not reliant on the Larentide Ice Sheet to distribute these materials during the last glacial.

...

The 70-80 km estimate is within range to derive this specimen from Mt. St. Helens.

 

"St. Helens which is ~50 miles(30km) north."

Thanks for the input Arc. My reservation is that the geological study of the underlying material and surrounding area does not mention glacial deposits/moraines as I would expect if that's what is there. As I have quoted a couple times now, the map legend says that the boulder is underlain first by Holocene and Pleistocene lake deposits, then Missoula Flood deposits under that. Also note, that your source says that during the last alpine glaciations the glaciers were 1/2 as long as the 70-80km mark and therefore the boulder seems unlikely to have been delivered by cascades glaciers to its present location.

 

Your source gives a map (Fig.2 Continued) wherein the closest 3 alpine glaciers are labeled Lewis, Adams, and Toutle (Toutle corresponding to Mt. St. Helens). By the map scale, Lewis is ~ 50km, Toutle ~60km, and Adams ~100km distant from the boulder's location. Unfortunately I can find no discussion of these 3 alpine glaciers in the text of the paper. Perhaps I overlooked it?

 

So again, and assuming the boulder doesn't extend 15 ft. underground, how is it that a 62 ton rock is sitting on the surface today? The choices as I see it are that it was dredged out and moved when the nearby lake was made, or it has been lifted by granular convection. i.e. frost heave. So far no one has commented directly on the possibility of the frost heave. Have I missed an option? Thoughts?

 

My idol idle curiosity is starting to boulder border on obsession. To scratch that itch I'm going to talk to the site manager who told me about the boulder in the first place and bring him up to speed on my investigations. I'll also propose to him that I contact the head of the local college geology department and see if I can tickle their interest, whether that be looking at the sample I collected for an ID, and/or coax them out to the site for a look-see. A proper identification of the rock ought to narrow down it's source for us at the least.

 

Don't we love a mystery!?

[arc]

Also note, that your source says that during the last alpine glaciations the glaciers were 1/2 as long as the 70-80km mark and therefore the boulder seems unlikely to have been delivered by cascades glaciers to its present location.

 

Your source gives a map (Fig.2 Continued) wherein the closest 3 alpine glaciers are labeled Lewis, Adams, and Toutle (Toutle corresponding to Mt. St. Helens). By the map scale, Lewis is ~ 50km, Toutle ~60km, and Adams ~100km distant from the boulder's location. Unfortunately I can find no discussion of these 3 alpine glaciers in the text of the paper. Perhaps I overlooked it?

 

 

I do not know exactly how far your specimen is from Mt. St. Helens. You did say in your first post it was 5 miles from the Columbia River and also St. Helens was 50 miles (30km) north.

 

Anyway, using an online calculator and using a volume of 728 ft³ (7'x8'x13') & presuming it is andesite as I surmised, the boulder would weigh approximately 125,000 lbs. (62 tons) and so by your own source could not have landed in its present location as a bomb from St. Helens which is ~50 miles(30km) north.

 

This rock could be from any of the Pleistocene glaciations and the earlier glacial episodes may have been very much different in their direction of travel than the more recent due to the amount of volcanic activity that drastically changes the surrounding landscape with each eruption.

 

 

As you can see St. Helens has been a very active player. So, if the 30km distance you provided is correct or the distance is even double that, the tremendous volume of material that the mountain has laid out around its range of discharge in the past could send in the following glacial periods, glaciers in any number of directions and distances within the stated ranges listed. This would seem the most likely source considering the evidence so far.

Edited January 28, 2017 by arc

[Acme]

I do not know exactly how far your specimen is from Mt. St. Helens. You did say in your first post it was 5 miles from the Columbia River and also St. Helens was 50 miles (30km) north.

That's as exact as we need.

 

This rock could be from any of the Pleistocene glaciations and the earlier glacial episodes may have been very much different in their direction of travel than the more recent due to the amount of volcanic activity that drastically changes the surrounding landscape with each eruption.

 

As you can see St. Helens has been a very active player. So, if the 30km distance you provided is correct or the distance is even double that, the tremendous volume of material that the mountain has laid out around its range of discharge in the past could send in the following glacial periods, glaciers in any number of directions and distances within the stated ranges listed. This would seem the most likely source considering the evidence so far.

Well, if you can find a geologic map or description showing/describing glacial deposits in Clark County Washington then I'll be the first to jump on board, particularly if they lie within 20 miles of the Columbia R. So far I have seen nothing of the kind on the maps or in the descriptions I have been reading.

 

Again, the big mystery for me is why the boulder is [apparently] sitting on the surface thousands of years after the presumed glaciers and documented giant floods. (Recall that the flood deposits are known to be ~15ft. below the boulder.) Can frost heave lift a 62 ton rock as it does smaller boulders, cobbles, and pebbles?

[Moontanman]

Frost heave could indeed be the culprit but a large earth quake could have lifted a rock through sediments to sit on the top. Take a can of pebbles of various sizes and sand, shake it up and watch the larger bits rise to the top of the can... It would be odd this is the only stone lifted in this manner...

[Acme]

Frost heave could indeed be the culprit but a large earth quake could have lifted a rock through sediments to sit on the top. Take a can of pebbles of various sizes and sand, shake it up and watch the larger bits rise to the top of the can... It would be odd this is the only stone lifted in this manner...

That is the granular convection I have been talking about, otherwise known as 'the Brazil nut effect'. I gave a link in post #3 and Wiki has an article as well. > Granular convection

Geology

 

In geology, the effect is common in formerly glaciated areas such as New England and areas in regions of permafrost where the landscape is shaped into hummocks by frost heave new stones appear in the fields every year from deeper underground. Horace Greeley noted "Picking stones, is a never-ending labor on one of those New England farms. Pick as closely as you may, the next plowing turns up a fresh eruption of boulders and pebbles, from the size of a hickory nut to that of a tea-kettle." [9] A hint to the cause appears in his further description that "this work is mainly to be done in March or April, when the earth is saturated with ice-cold water". Underground water freezes, lifting all particles above it. As the water starts to melt, smaller particles can settle into the opening spaces while larger particles are still raised. By the time ice no longer supports the larger rocks, they are at least partially supported by the smaller particles that slipped below them. Repeated freeze-thaw cycles in a single year speeds up the process.

 

This phenomenon is one of the causes of inverse grading which can be observed in many situations including soil liquefaction during earthquakes or mudslides. Granular convection is also exemplified by debris flow, which is a fast moving, liquefied landslide of unconsolidated, saturated debris that looks like flowing concrete. These flows can carry material ranging in size from clay to boulders, including woody debris such as logs and tree stumps. Flows can be triggered by intense rainfall, glacial melt, or a combination of the two. ...

What I didn't consider, and you so astutely point out, is that an earthquake can produce the effect as well as frost heave. Nice! This immediately put in mind the last Cascadia quake which occurred in January of 1700, estimated at a magnitude between 8.7 and 9.2.

The field floods every year and would have been saturated at that time, and voila, lifted boulder. I like it. Moreover, the Cascadia fault ruptures at intervals of several hundreds of years:

There is evidence of at least 13 events at intervals from about 300 to 900 years with an average of 570590 years.[16] Previous earthquakes are estimated to have occurred in 1310 AD, 810 AD, 400 AD, 170 BC and 600 BC.

The boulder may have been lifted all at once, or in stages during different quakes. During the summer the prairie dries out and so one wouldn't expect the effect in that season.

 

As to this boulder being the only one of its size I cannot attest. While it is the only one this large in the immediate area, there are over half-a-dozen historic wet prairies in the county and there may well be more such monoliths. Since we are overdue for the next Cascadia quake, if it comes in my lifetime I may get to witness the emergence of more boulders. A bright spot in what is going to otherwise be a monumental disaster.

Thanks for the input TanMoonMan!

[Acme]

I had some plant cataloging to do in the field today so I took time to photograph some of the typical boulders in the area. The stick is 42" long for scale. The pile is the result of past farming efforts, which were never successful at this location beyond grazing stock. The stock grazing began with the establishment of Fort Vancouver in the early 1800s and ended about 20 years ago.

 

[studiot]

Nice to see folks cooperatively working through the possibilities with open minds. +1Acme and Mman

 

Edited January 29, 2017 by studiot