 |
| Three little lakes in the cirque below Piegan Pass |
 |
| Piegan Pass |
A knife-sharp crest, the continental divide, cuts Glacier Park
down the middle. It slices snow and rain
as they fall, sending some to the Pacific Ocean, the rest to the Gulf of
Mexico. I sat on a low point in the
divide, called Piegan Pass. There I
propped myself against an intrusive igneous rock, and looked across a cirque at
a vertical wall of uniform flat layers—Precambrian limestone—and wrote notes
about the day so far.
 |
| Nonconformity above the lake in the right of this picture |
 |
| Close-up of the nonconformity |
I noticed a gap in the sedimentary layers, a nonconformity in
the regular buildup of limestone; it appears to be filled with something
else. A light-colored layer angles up
and through a dark layer of limestone.
Though solid and stationary today, something seems to have pressed in
from the left, punched through the dark layer above it, and then proceeded
onward to the right. But is that what
really happened?
 |
| Standing on the light-colored layer |
 |
| Close-up of the light-colored layer |
Come, you geology buffs; I know you’re out there. A prize of inestimable worth goes to anyone who can explain the nonconformity.
 |
| Close-up showing crystalline structure in the light-colored layer |
 |
| Mount Siyeh, 10,014 feet, looking down on us, judging our answers |
Come theologians, spiritualists, poets, and artists; speak where geologists equivocate and where broad-sweeping theories sometimes teeter on details that don’t fit. Nature has thrust on us a puzzle in a rock wall. I’d like your reactions to the evidence set before us. I think I hear a giggle coming from the wall, a sound awaiting an answer.
“An artist is an
instrument of unknown energy.” - Susan Dobay, from her new book.
Wish I could say something smart, but I havent't got a clue about the puzzle.
ReplyDeleteYou just did say something smart, Mary
DeleteMany mysteries in the rocks
the sky and sea
let them linger
or say we know
the rocks--
indifferent either way
Hi Sharon,
DeleteI will do some digging on this, but right off the top of my head of very little knowledge, here's a guess. This could be a granitic sill. It's not an unconformity as no time was lost. It looks from your pictures that the mountain's sediments were laid down in a shallow ocean and as the waters rose and got deeper limestone was laid down by coral reefs, and as the sea level dropped fine sediments were deposited that became shale. So where the side of the mountain range is a cliff it is limestone which is resistant in a dry climate and where the mountainside is sloped because of erosion it is shale which is non-resistant. At some point in the mountain building of all this it looks like magma came up and intruded into the shale and followed the bedding. I guess it would have started out as a dike and then went parallel to the bedding and became a sill. The magma would have then slowly cooled into some kind of granite type rock. Your photographs are absolutely fantastic. I'm so glad you area able to experience all this and bring it to the rest of us. Thank you so much.
Ellen, Thank you for this insightful answer. I think it is the most geologically correct of the several answers I have received; and you have the bravery to post it publically, while others have answered by email. I am still waiting for poets, theologians, spiritualists, and artists to come forth with what might be just as valid insights. I am not an expert, but will join you in “further digging.”
ReplyDeleteThe Precambrian limestone structure in these photos is part of the Lewis Overthrust of 60-70 million years ago, which comprises most of Glacier National Park. Some good overview sites are: https://www.nps.gov/glac/learn/education/geology.htm https://www.nps.gov/features/glac/resources/geology.htm http://pubs.usgs.gov/pp/0294k/report.pdf and also those pages from a book you gave me, which I don’t have the author and title of.
In summary of these: 1.6 billion to 800 million years ago, a variety of materials were eroded and washed into the ancient Belt Sea, forming deep sediments of differing composition, including limestone. Heat and pressure has given crystalline structure to some of the layers, leaving them mostly level and without making them look metamorphic.
750 million years ago, an intrusion of magma rose into the mostly level layers, forming dikes, which fed sills of magma spreading out between the layers. The Purcell Sill is known to run through the general area of these photos. It is described in the literature as a dark band of igneous diorite about 100 feet thick running through the Siyeh Limestone, on which I believe I was standing for these photos. Mt. Siyeh, shown in one of my photos, is one place where it has been identified. The heat of this intrusion is said to have crystallized the limestone, making it look igneous in some places.
Perhaps the dark layer we see in the photos is not ancient limestone, but rather the Purcell Sill. And perhaps the crystalline light-colored layer is not igneous, but recrystallized limestone.
The puzzle, to me, is the “S” shaped, light-colored layer, where it runs through, apparently cutting, the dark-colored layer.
I remain 'puzzled' but then I find a comfort in that ~ so nice to sit back and watch the minds gather to unravel the mysteries, share the concepts, ponder the casualness of 'millions and billions and trillions' as if they were just one yesterday away. And, so, me thinks, 'tis so.
ReplyDeleteIt's nice to sit back and
Deletewatch the unraveling of mysteries
share the concepts
ponder the casualness
of billions of years
as if they were one yesterday
Thanks Junnie for settling my mind. It gets confusing without some poetry to calm me down.
Here's a wild guess: a layer of salt, perhaps? Encrusted over time ?
ReplyDeleteErika
You're a wild woman, Erika. I like that.
Deletea poet's perspective
ReplyDeletelong ago a giant white animal
Glacier, rolled through these hills
melted
thousands of years
to tiny lakes their pawprints
do deer come sipping?
tiny lakes
Deletepawprints of glaciers
where deer come sipping
they welcome you back
from glaciated chicago