[Rockhounds] glacial erratics (was Stonehenge geology resolved)

[R. Peter Richards]

Since it is a sedimentary rock (albeit perhaps a metamorphosed one),  
the details of the thickness and texture of the beds should answer  
this question.  If both pieces are part of the same rock originally,  
the bed thicknesses and grain sizes should match closely from one to  
the next across the split.  If they are just two rocks that happened  
to wind up next to each other, there is no reason that these  
characteristics would match nearly as well, and they might not match  
well at all.  Unfortunately, one cannot judge this from the  
photograph, but I would have to guess that someone has applied this  
test.

Pete Richards


On Feb 2, 2009, at 12:13 PM, Lanny R wrote:

> I have a doubt about the "Big Rock," this Okotoks Erratic really  
> being one rock broken into two pieces. First, I don't recall ever  
> seeing a large rock that I was sure had obviously broken into two  
> pieces where it had settled in the middle so that the two pieces  
> dipped inwards. Secondly, I can't see any match in the layers  
> between the two pieces. That might be just a problem of how it is  
> shown in the photograph, so that one can't really see them all that  
> well. Thirdly, the piece on the right is more broken up and has a  
> lot of  rusty layers, very unlike the left piece.
>
> Any others care to express their opinion on this?
>
> Regards,
>
> Lanny
>
>
> On Feb 2, 2009, at 8:48 AM, pmodreski at aol.com wrote:
>
>> Neat website about that big rock, Kreigh!
>>
>>
>> Some erratics are big. You might enjoy the size of the biggest one?
>> ?
>> http://culture.alberta.ca/museums/historicsiteslisting/ 
>> okotokserratic/default.aspx?
>> ?
>> and the interesting legend of how it split in two.?
>> ?
>> Kreigh?
>>
>>
>> And, alas, I'm afraid your post was a great disillusionment to me,  
>> because I couple of years ago I had visited, and was very  
>> impressed by, the Madison, Carroll County, New Hampshire, glacial  
>> erratic, a N.H. State "Wayside Monument" and locally known as "the  
>> largest known glacial erratic".? It is,
>>
>>
>> "Madison Boulder is a huge granite rock measuring 83 feet (25 m)  
>> in length, 23 feet (7.0 m) in height above the ground, and 37 feet  
>> (11 m) in width. It weighs upwards of 5,000 tons."
>>
>> Your Okotoks Erratic "Big Rock" in Alberta has?clearly got it all  
>> beat,
>>
>> "The Okotoks Erratic weighs 16,500 tons. It measures 9 metres  
>> high, 41 metres long and 18 metres wide."
>>
>> and?evidently holds the world record, as is stated in the very  
>> good Wikipedia article about glacial erratics,
>>
>> http://en.wikipedia.org/wiki/Glacial_erratic
>>
>> Next time I'm in Alberta (never been there, actually), I've got to  
>> see it!
>>
>> Cheers, Pete
>>
>>
>>
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___________________________________
R. Peter Richards
rpr at heidelberg.edu
Morphological crystallographer





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