Fascinating mountain rocks

Thread Starter

cmartinez

Joined Jan 17, 2007
6,601
I'm back from my weekend trip to the mountains... and while I was taking a long walk, enjoying the beautiful scenery, I stumbled across a few of this strangely shaped, razor sharp rocks:


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The place in question is located about 2,800 meters above sea level (9,200 feet). Here's a google earth link to the place.

What they all have in common is the deep grooves and cuts on their faces. But some of them had semi-sharp features, and yet others were far smoother and rounder. Their average size was about 1 x 1.5 x 2 meters.
Also, most of them were bundled in groups. There were also about five or six groups laying around that had sunk in the ground, into dangerous-looking sinkholes.


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This is my theory: These rocks are remnants of the last ice age. Some of them show the fossils of shells belonging to clams, oysters and mussels. They are sharp because they were carved by a glacier while it melted and retreated from the valley. Ice is harder than rock, as the ice shifted, it broke and scratched the rocks into their current shape. Creeks through which the water flowed were formed as the ice melted, and that explains the erosion and roundness of all the other rocks. As all of the ice finally melted, pools of water were left behind, forming ponds into which some of the rocks slid and ended up as the sinkholes that can be seen today.

I'd like to hear other people's opinion on this issue. I find geology a fascinating subject, but know too little about it.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
6,601
???????

Ice is about 1.5 on the Mohs scale. Talc is 1 and Gypsum is 2. Also in the 1.5 range is lead and graphite, both of which are used as lubricants or anti-friction surfaces.
So how does it manage to do that to rock? Is it because it's more abrasive?

Then again, isn't abrasiveness directly related to hardness?

Or is there some other mechanism at play? Maybe extremely low temperatures make rock more fragile?

I'm almost certain those rocks are not volcanic in origin. I find no other explanation for them ...
 

Thread Starter

cmartinez

Joined Jan 17, 2007
6,601
???????

Ice is about 1.5 on the Mohs scale. Talc is 1 and Gypsum is 2. Also in the 1.5 range is lead and graphite, both of which are used as lubricants or anti-friction surfaces.
I've just found out that the hardness of ice increases as its temperature decreases... it's 6 Mohs at -70C ...

http://academic.emporia.edu/aberjame/ice/lec02/lec2.htm

As a reference, quartz has a hardness of 7
 

WBahn

Joined Mar 31, 2012
24,854
So how does it manage to do that to rock? Is it because it's more abrasive?

Then again, isn't abrasiveness directly related to hardness?

Or is there some other mechanism at play? Maybe extremely low temperatures make rock more fragile?

I'm almost certain those rocks are not volcanic in origin. I find no other explanation for them ...
There are many erosion mechanisms. The ice, particularly at the bottom, likely has lots of debris entrained in it. There is also the grinding of the rocks against themselves (and whatever else) as they are transported by whatever mechanism does so. Then there is the freeze-thaw cycle that is extremely effective (in suitable environments) for fracturing rocks. There can also be chemical reactions or dissolution with the water or what is in it. Plus, scraping something soft against something hard DOES remove material from the harder surface as well (otherwise knives and other cutting edges would almost never need to be sharpened). Given a few million years, it becomes quite evident.
 

OBW0549

Joined Mar 2, 2015
3,028
This is my theory: These rocks are remnants of the last ice age. Some of them show the fossils of shells belonging to clams, oysters and mussels. They are sharp because they were carved by a glacier while it melted and retreated from the valley. Ice is harder than rock, as the ice shifted, it broke and scratched the rocks into their current shape.
When glaciers carve grooves into rocks, it is not the ice that does the carving but rather the stones and boulders embedded in the ice at the base of the glacier. Ice is nowhere near hard enough to do the job itself, not at any earthly temperature anyway-- even during an ice age.

Also, the grooves left in rocks by glaciers tend to be long, straight and parallel, and oriented in the direction of glacial ice flow. A good example near me is in Central Park in NYC, which has a lot of grooved rocks (see here, here and here). The grooves in your rocks don't fit that description; they're short, curved and in all different directions. I don't know what caused them, but it wasn't a glacier.

Did you even get glaciers down there? I wasn't aware that any of the major North American glacial ice sheets got down that far.
 

shortbus

Joined Sep 30, 2009
7,091
I live in an area that is known as a "glacial kame" an area that the rocks and soils were left behind in by a retreating glacier. Our rocks are mostly rounded due to movement in the glacier, rocks rubbing and tumbling against each other. so kind of don't think that's what caused those rocks to look like that. Any idea of what type of rock they are?

https://en.wikipedia.org/wiki/Kame
 

philba

Joined Aug 17, 2017
960
If there are fossils in the rocks then they are sedimentary and probably not that hard. I think glaciation would have pulverized them.What kind of local wildlife is there? Is it possible the grooves are human in origin? Spear or knife sharpening?
 

Thread Starter

cmartinez

Joined Jan 17, 2007
6,601
Also, the grooves left in rocks by glaciers tend to be long, straight and parallel, and oriented in the direction of glacial ice flow.
There are rocks aplenty like that down here. In fact, most mountains have layered patters, which have a clearly sedimentary origin. The most extreme can be seen in the Huasteca mountains, in which the layers have been turned completely vertical due to geological shifting.

Any idea of what type of rock they are?
They look like what we call ordinary "river rocks", except for their sharp cuts. And no, I have no idea what to call them.

Lava in water.
They're not that kind of rocks... lava rocks are generally porous and these are dense and solid. Besides, there are no ancient volcanoes nearby.

Is it possible the grooves are human in origin? Spear or knife sharpening?
No way... these are very big rocks... I should've called them boulders instead of just rocks.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
6,601
Did you even get glaciers down there? I wasn't aware that any of the major North American glacial ice sheets got down that far.
I was told by a teacher, many years ago, that the ice sheet in this latitude reached a thickness of about 1,200 meters ... but I have never verified if it was in fact that way... gonna do some searching and find out.
 

OBW0549

Joined Mar 2, 2015
3,028
I was told by a teacher, many years ago, that the ice sheet in this latitude reached a thickness of about 1,200 meters ... but I have never verified if it was in fact that way... gonna do some searching and find out.
I'll be really surprised of that turns out to be true.
 

WBahn

Joined Mar 31, 2012
24,854
I'm trying to figure out the alternative of how they could have migrated from Asia to North America by traveling through the interior?

I can only imagine that what they are talking about is that, once arrived in North America, that they traveled down the coast from the point of entry as opposed to immediately traveling inland first.

But I don't see how this find supports that in any way. They found a couple dozen footprints in a small area. Was this near their point of entry onto the continent? If so, how do they know that they didn't travel inland from there? Even if they determine that THIS group of three people came down the coast from further away, what does that prove? Maybe they and others from their group spread out up and down the coast fishing and gathering food in preparation for their intended journey inland. Even if THIS group of people (and everyone in their large group) migrated down the coast, that in no way says that EVERYONE one, or even the majority of people, did so. It also does not establish that a huge wave of migration didn't come across hundreds or even thousands of years earlier and set off inland long before this tiny group of people arrived and lingered (or possibly stayed permanently) around the coast for some amount of time.

I understand that archaeologists have very little data to work with and have no choice but to make pretty sweeping extrapolations, but I find it annoying how it is always presented as divined fact. In fairness, a lot of that is due to the media. When you look at the writings of these folks directly they are much more cautions in their pronouncements and careful to talk about what such and such might indicate. But the media is more often than not insistent on turning them into proclamations of indisputable facts.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
6,601
I'm trying to figure out the alternative of how they could have migrated from Asia to North America by traveling through the interior?

I can only imagine that what they are talking about is that, once arrived in North America, that they traveled down the coast from the point of entry as opposed to immediately traveling inland first.

But I don't see how this find supports that in any way. They found a couple dozen footprints in a small area. Was this near their point of entry onto the continent? If so, how do they know that they didn't travel inland from there? Even if they determine that THIS group of three people came down the coast from further away, what does that prove? Maybe they and others from their group spread out up and down the coast fishing and gathering food in preparation for their intended journey inland. Even if THIS group of people (and everyone in their large group) migrated down the coast, that in no way says that EVERYONE one, or even the majority of people, did so. It also does not establish that a huge wave of migration didn't come across hundreds or even thousands of years earlier and set off inland long before this tiny group of people arrived and lingered (or possibly stayed permanently) around the coast for some amount of time.

I understand that archaeologists have very little data to work with and have no choice but to make pretty sweeping extrapolations, but I find it annoying how it is always presented as divined fact. In fairness, a lot of that is due to the media. When you look at the writings of these folks directly they are much more cautions in their pronouncements and careful to talk about what such and such might indicate. But the media is more often than not insistent on turning them into proclamations of indisputable facts.
Bahn, I suggest you take a look at 1491 and 1493 (the latter is more fun), by Charles C Mann. Both are fascinating reading dealing with the subjects you've just mentioned.
 
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