29 August 2015

Mystery structure: please help identify / interpret

I have a mystery for you today:




These are samples of Tonoloway Formation carbonate (not sure if it’s limestone or dolostone in retrospect), with bedding more or less horizontal in these images, and a few petite stylolites running orthogonal to that. The top sample has a gentle fold 2/5ths of the way across. All of the samples are from the same site in West Virginia, along Corridor H.

I’m wondering about the closely-spaced vertical (bedding-perpendicular) lines. Have any of my readers observed these before? What are they? How do they form?

Thanks in advance for any insight you can share.

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21 August 2015

Friday fold: a kink fold at Baldwin Gap

Ladies and gentlemen, I give you…

The Friday fold:


This beauty came to my attention on Monday, when I was lucky enough to go on a field trip with my friends Leigh and Mary. They are founding members of our local informal geology club, and we have been meaning to take a Cedar Creek field trip together since I moved out to the Fort Valley.

One of the sites we visited is a roadcut along the southern side of Baldwin Gap, a small water/wind gap in Little North Mountain.

While at Baldwin Gap, the question they posed to me was whether siliciclastic strata had been thrust on top of carbonates, and what unit was what.

We parked near the top of the hill (east) and walked downhill (west). We puzzled over what we were looking at – Pseudostenography of the conversation:

Was that Martinsburg turbidites?

No, can’t be Martinsburg, because here’s a quartz sandstone.

Wait, here are some redbeds – that looks like Bloomsburg. Hmmm – what unit could this be?

Sure enough, here at the bottom of the hill are limestones, including mudcracked limestones, which suggests Tonoloway Formation. Is this one unit? Two divided by a thrust fault?

…more than two…?

As far as structures go, we noted several folds (the one above was best expressed) and two small-offset reverse faults, with westward dips.


A field experience like that, for which I had not specifically prepared, raised questions in my head, and I went home eager to answer those questions.

Fortunately, the area has already been mapped.

The Geologic Map of the Middletown, Virginia quadrangle (1999) by Orndorff, Epstein, and McDowell was the resource that confirmed our field observations and put them into a regional framework. Here’s a screenshot I took, annotated with key units, and including a detail of the thrust-faulted geology of the gap itself:


The ridge into which the gap is cut hosts not one, not two, not three, but four separate blocks, stacked up along thrust faults. This explains the diversity of llithologies we saw, and –delightfully– confirmed our assessment of which units we were seeing. Sure enough, there were clastics at the top of the hill (east), thrust on top of limestones at the bottom (west). The map-scale faults dip east, so the small-scale faults we observed were antithetic to that orientation – not an uncommon thing to observe.

The fold is probably in Tonoloway (or Helderberg?) – it was pretty much the westernmost outcrop in the gap.

This outcrop is worth sharing with the world, so in the fall or winter, when the vegetation dies back a bit, I’ll revisit it with my GigaPan.

Thanks for showing me this place, Mary and Leigh!

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20 August 2015

Stromatoporoids in a Devonian reef

Here are a few shots of a Devonian aged reef exposed in Mustoe, Virginia – one of the sites I visited this spring with GMU’s Rick Diecchio, when he led his sedimentology and stratigraphy trip there.

At first, the outcrop made no sense to me – I kept searching for bedding, and failed to find it. Then, the reef interpretation clicked, and suddenly I didn’t “need” bedding any more…






Stromatoporoids were probably spongelike organisms, with layers like stromatolites, but in those layers they have a tiny architecture of ‘rooms’ and ‘pillars’…




A lot to see here – certainly a site I should return to some day with a GigaPan.

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19 August 2015

Iron oxide sweat stains in sandstone

Have you ever found a rock sample that looked something like this?


It’s got a lot of rust, but mainly around the edges. What’s up with that?

Check out my hat for a clue – this is after a long hot afternoon picking apples in the sun on Sunday:


Did you see the dirty “rind” where my sweat soaked in and moved dirt and grime along with its flow, to accumulate at the edge of the soaked zone? Flow paths depicted in the annotated version below:


In the same way, fractures (joints and faults) in rock can serve as conduits for fluid flow, and surfaces along which subterranean fluids can soak into the “fabric” of the surrounding rock.



In the case of this sandstone, the white is “unsoaked” and the brown shows the rusty signature of overlapping successive “soaking fronts”:


All the rock examples in this post are from the new Bismarck, WV, outcrop of Conemaugh Group sedimentary rocks exposed along Corridor H. Here’s an example of conglomerate overlying sandstone, with both hosting the Liesegang-like iron oxide banding:



Zooming in on the contact:


These features can be quite beautiful, making Rorschach-blot like patterns in the rock…


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18 August 2015

The Revenant, by Michael Punke

On the recommendation of my friend Troy Holland, rev I just finished reading Michael Punke’s novel about Hugh Glass. The book has been optioned as a film, and because it stars Leonardo diCaprio, it will doubtless be a hit. What got Troy’s attention, though, is the director: Alejandro González Iñárritu, the guy who gave us Babel, Birdman, and Amores Perros, all of which were masterful films.

So what’s a ‘revenant?’ Wikipedia’s got it: “A revenant is a visible ghost or animated corpse that was believed to return from the grave to terrorize the living. The word “revenant” is derived from the Latin word, reveniens, “returning” (see also the related French verb “revenir”, meaning “to come back”).”

In this case, Hugh Glass is the revenant, but as an analogy. Glass, a real person, was mauled by a grizzly bear while in the employ of the Rocky Mountain Fur Company in 1823. Two men were assigned to watch over him while the main body of the party went upriver, and to bury Glass when he inevitably died. Except they didn’t – they abandoned him. Glass didn’t die. Deprived of almost every resource, he vowed to survive and visit vengeance upon the two men, one of whom was Jim Bridger (namesake of my beloved Bridger Range near Bozeman, Montana).

His odyssey is insane – the difficulties that Glass overcame, expertly described (in lightly fictionalized form) by Punke, are astonishing. Rattlesnakes, starvation, blizzards, Arikara warriors, and even federal bureaucracy all conspire to put an end to Glass’s quest for revenge. But he keeps working at it, keeps coming up with clever ways around problems, like a frontier MacGyver (though a bloodthirsty one).

The book is a great way to revisit the post-Lewis-&-Clark, pre-civlization days of the Rocky Mountain west. Punke’s narration brings to life many of the daily routines, risks, and perspectives of ‘the white man” entering a land he saw as savage. Native Americans will probably not be thrilled with the rendering of the Arikara as savages, however. They fought tenaciously to keep white pioneers from invading their homeland, but The Revenant depicts that all from the pointy end of the arrow – little sympathy is conveyed, which I think is an accurate choice, in depicting the events of the novel from Glass’s perspective. There are a few token “good Indians” in the story, but none of them have their backstories explored, as a half dozen of the non-Glass white charaters do. Instead, the more dominant treatment for the Native Americans is straight up “cowboys and Indians” (good guys and bad guys). So that’s that.

Overall, it was a compelling adventure story – although I’ll warn you that the ending didn’t really satisfy me as much as I had hoped. I’m looking forward to the movie. Here’s the trailer:

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17 August 2015

Geology Underfoot in Western Washington, by Dave Tucker

uw I’m grateful to Mountain Press for sending me copies of all of their new books. There are some terrific volumes that have arrived in my mailbox over the past year, and I feel guilty for not reviewing more of them. But when I upwrapped this one, I was struck by two things:

1) The author is a geoblogger, and a prolific one. Dave Tucker writes Northwest Geology Field Trips, and apparently that effort led to the authorship of this book. That piqued my interest.

2) The cover image is an extraordinary image of Mount Rainier sending a lahar down to Puget Sound – beautifully rendered and as clear a blend of geoscience and art as I have ever seen.

So I read it.

The website seems like an incredible resource for western Washington geology, and the book is the same. There are some really amazing sites detailed in the book – places I would love to see. Mr. Tucker writes about them very clearly and compellingly. I read it cover to cover and learned a lot. It’s beautifully produced, with high quality graphics and color photography, and the analogies are excellent.

The one thing that drives me bonkers about the website is the ‘stamping’ of all the photos with an inelegant source ‘watermark’ (example). I understand why Mr. Tucker does that – internet plagiarism is rampant and he’s only being proactive in protecting his imagery – but it’s such an ugly way of claiming one’s photos as one’s own. To the aesthetically-inclined eye,  the watermark really distracts from the geologic content of the imagery, rendering the photo less useful than it would have otherwise been. To me, it really gets in the way.

That’s not the case for the book, which allows a reader to immerse themselves as directly as possible in the geology of these diverse sites, with few reminders that Mr. Tucker is there acting as translator (meaning he does a very good job). Clastic dikes, pillow basalts, glacial erratics, lahar deposits, ghost forests, waterfalls, spits – there’s a wealth of geology in western Washington, and this book is THE resource I would use to guide me in exploring it.

I recommend this book if you’re on your way out to western Washington anytime soon.

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15 August 2015

Paleobiology database survey request

Dear colleagues,

My collaborators and I are exploring the potential of big science databases, like the Paleobiology Database (see below), to enhance geoscience education and research at all types of institutions. We’re very interested in learning who is and isn’t currently using this and other databases for education and research and why. This work is supported by an NSF DUE grant #1504588.

If you could take about 10 minutes to fill out the following survey, we’d very much appreciate it:
If the link doesn’t work, please feel free to copy and paste the web address.

The Paleobiology Database (PBDB) is an online database that stores information on when and where fossils occur throughout geologic time. It includes a variety of information such as taxonomy, abundance, ecological traits, and body size, all gathered from the scientific literature. You can find out what the PBDB has to offer here: www.paleobiodb.org.

If you have questions regarding this survey, please feel free to contact Dr. Rowan Lockwood at the College of William and Mary (rxlock@wm.edu).

Callan Bentley (Northern Virginia Community College)
Peter Berquist (Thomas Nelson Community College)
Christian George (High Point University)
Rowan Lockwood (College of William and Mary)
Laura Lukes (George Mason University)
Katherine Ryker (Eastern Michigan University)
Mark Uhen (George Mason University)

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14 August 2015

Paleoslump features and fluvial incision in the Conemaugh Group, West Virginia

For the past two days, I’ve been asking readers to think about some interesting rocks exposed along a newly-opened stretch of Corridor H in West Virginia.

Here’s the scene.


I visited the site twice last week – once with Dan Doctor (USGS), Mitch Blake (West Virginia survey), and Alan Pitts (University of Camerino), and then again with Jeffrey Rollins (Old Dominion University & Team GEODE) and Alan Pitts again.

We are talking about these rocks:

dragonstongue_south_ice-25_sm Click to enlarge
dragonstongue_south_ice-25anno_sm Click to enlarge

Today is the big reveal – a detailed look at this site. But the images I showed you above were only a taste. The real action is across the street:


Whoa! That looks like an angular unconformity, right? Check it out in these GigaPans:

link [GigaPan by Marissa Dudek]

link [GigaPan by me]

But here it is in broader context:

link [GigaPan by Jeffrey Rollins]

link [GigaPan by Alan Pitts]

A detail of the central area showing coal seams in crazy orientations (Mitch Blake christened this site “The Dragon’s Tongue”):
link [GigaPan by me]

I hope at this point you have noticed all the different angles at which bedding appears in these roadcuts, and the various truncating relationships between key marker beds. Perhaps, you are wondering what’s going on here.

These strata are Pennsylvanian in age (later Carboniferous). They are part of the Conemaugh Group.

These strata are sandstone, siltstone, shale, and coal. The coal is the black stuff:


Here are two coal seams, at a ~20º angle to each other:


As you might expect with all that coal, there are lots of plant fossils in these rocks:



Here are two samples of the distinctive diamond-shaped pattern seen in lycopsid bark:



Here’s a section of stump/branch/log/root that Jeff found:


A stem in one of the white sandstones:


And another example of big pieces of coalified wood in the coarse, white sandstone:



So, what’s going on here? Another example of a truncated coal, ending abruptly (and at an angle) relative to an overlying sandstone (here, stained orange from rust):


The bottom of this same sandstone shows features that look strongly like “ball and pillow” (soft sediment deformation due to density inversion):




So maybe these relationships are sedimentary, depositional.

But… the angles of bedding are all over the place! Look at this:



Could these layers be juxtaposted through tectonic deformation instead? Could these abrupt truncations be thrust faults? Certainly that was a common theme in the guesses offered by readers.

There is some smaller-scale structure to be observed, such as this batch of small crinkles in on of the shale units:


…or these recumbent folds in siltstone layers, a few inches away:



In one of the highly tilted sandstones, there is internal folding, too:




Here, the trace of bedding can be followed around the left and lower edges of this block weathered out from the outcrop, apparently overprinted by a weak crenulation lineation:


Look at this truncating relationship:



Jeff examines this site (same site; photographer’s perspective has shifted a few meters to the left):



Elsewhere, there are odd “lozenges” of coal in the sandstone:



Is that a coalified branch? Is it a bleb of former peat that got torqued and twisted during deformation? Was the deformation pre-lithification (and coalification), or post-?

One idea on how to interpret these rocks is that there these discontinuities represent tectonic (Alleghanian) faulting, but when we looked for slickensides, fault breccia, etc., we didn’t find it. Instead, we think the best way to interpret this outcrop is a series of paleo-slump-blocks that were then incised by rivers, removing some portion of the slump blocks and depositing coarse channel sandstones.

Here’s Alan Pitts’s annotation of the outcrop, based on a hand-held panorama (one of the GigaPans embedded above) and condensed a bit by me in order to economize on space:

annotated_Conemaugh_AP_CB_mods_sm Click to enlarge

This I think, is instructive on two levels: (1) Alan is a talented graphic artist, and (2) this sort of annotating a high-res image is a useful method of developing hypotheses about an outcrop. You can see, for instance, that Alan has outlined the prominent white sandstone blob in the upper left with two faults. Here’s a close up of that site from the upper bench:



It didn’t look like it was fault-bound to me; it looked to me like a channel incising into (slump-) tilted strata, and then filled in two separate episodes (earlier dark gray and medium-grained sand, later white and coarse-grained sand). In other words, in my mind, the arcuate bottom to that protrusion is primary, a depositional feature, not a later tectonic imposition.

So here’s my attempt at annotating Alan’s GigaPan (i.e. same base imagery as above, but different a geologist making the lines and interpretations, using a stylus on a Wacom Cintiq):

annotated_Conemaugh_CB-smClick to enlarge

I did that solo, without reference to Alan’s annotations, to minimize his influence on my thinking, and see what I would come up with on my own. He may have missed some stuff I caught, and I may have missed some of the stuff he caught. Caveat emptor: Both Alan’s sketch and mine are preliminary – a method of practicing looking at the outcrop. Nothing here should be taken as conclusive. I share them here in their unpolished state to reveal the way we approached this problem, and how the imagery was a catalyst to our thinking.

I interpret these rocks as paleoslump blocks – layers of clastic sediment and peat laid down, perhaps during eustatic highstands, then incised during the regression accompanying a late Paleozoic glaciation. These incisions likely oversteepened the slopes of these soggy sediments, and they sloughed into the resultant valleys. This rotated the strata, and exposed the leading edge to erosion at the river’s cut bank. The slump blocks, as Alan pointed out, would have a “normal” sense of offset on their trailing edge, but a compressional (and potentially even “reverse”) sense of kinematics on their noses.

Later, transgression accompanied deglacation, and  fluvial deposition laid down the coarse white cross-bedded sandstone (with channel lags of gravel and big branches of plants included). Then another cyclothemic cycle went into force, and the pattern repeated. That, to me, is the most economical way to explain what I see at these outcrops. But I’m open to being convinced otherwise: You now have the same data we do. Take a crack at it – and once, again, Tell Me What You See Here….

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12 August 2015

Rockies student projects, 2015

Another year, another batch of student projects from my Rockies field course, each intended to elucidate some aspect of the geology of the Montana / Wyoming Rocky Mountains for the general public:

  1. Geology of Grand Teton National Park (Marcell)
  2. Tilted glaciolacustrine beds near Glacier National Park (James)
  3. Blog on various aspects of the field course’s geology (Zack)
  4. A geologist’s guide to the Beartooth Highway (Peri)
  5. The Sawtooth Range and the Disturbed Belt (Leslie)
  6. YouTube video on the Purcell Sill and Helena Formation stromatolites (Nathan)
  7. The geology of hiking the trail to Grinnell Glacier (Lauren)
  8. The geologic story of the Bridger Range (Anna)
  9. Grinnell Glacier (Hannah)
  10. Grinnell Glacier hike (Emily)
  11. Sun River Canyon (Matthew)
  12. Sun River Canyon (Kathlene)
  13. Ringing Rocks Pluton (Theresa)
  14. Basement Complex (Jake)
  15. Basement Complex (Xiuming)
  16. Absaroka Volcanic Province (Brad)
  17. Boulder Batholith, Ringing Rocks, and the Lewis Thrust (Karisha)
  18. Geothermal Features of Yellowstone (Travis)
  19. The Belt Supergroup in Glacier National Park (John)

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Student guest post: the Belt Supergroup in Glacier National Park

As longtime readers know, late summer is when my Rockies students submit their final projects – web-based explanations of key geologic sites they examined during the trip.

Today, I offer you a guest blog post by student John Leaming. You’ll notice that I’m not *completely* absent from the post, however – I make a couple of cameos as “sense of scale.”




Glacier National Park, Belt Supergroup

I recently traveled through Glacier National Park as part of a Field Geology Course of the Northern Rocky Mountains. The distinctive and very distinguishable strata observed throughout the park’s landscape showcase the mainly sedimentary rock that comprises the Belt Supergroup.
Click here for a map of Glacier National Park, courtesy of National Park Service.

The Belt Supergroup which comprises most of the mountains in Glacier National Park, is a thick series of sedimentary rock layers that were formed from an ancient passive rifting event about 1.6 to 1.4 Ga, where the Siberian Platform separated from the North American Craton, forming a massive Mesoprotozoic age sea basin known as the Belt Basin or Belt Sea. This basin would become the depositional location for an immense amount of sediment deposited from the surrounding landscape. Due to mass wasting and erosion, the rapidly subsiding Belt Basin was able to accommodate more than 15,000 ft. of sediment deposited ~1.47 to 1.4 Ga. Sediments such as beach sands, shallow water and fine sands, mud, silt, and limestone from varying depositional environments within the basin at the time, formed the distinctive rock layers we see today in the Belt Supergroup.

Image from formontana.net


The Formations of the Belt Supergroup

The sedimentary rock formations within Glacier National Park can typically be distinguished by their color. From oldest to youngest, the formations are: Prichard and Altyn, Appekunny, Grinnell, Empire, Helena or Siyeh, Snowslip, and Shepard.

  • The Prichard Formation is found in the western portions of the park and is ~1.375 to 1.4 Ga siltstone and dark-gray argillite, which is a lightly metamorphosed mudstone, and is ~4,000 ft. thick. These rocks are roughly the same age as rocks found in the Altyn Formation, but were deposited in a deeper low oxygen water environment, resulting in its darker color.
  • The Altyn Formation is found in eastern portions of the park and is composed of ~1.350 to 1.450 Ga limestone and dolomite ~780 -840 ft. thick. They are light-gray or have been weathered to a light tan.
  • The Appekunny Formation is composed of ~1.375 to 1.4 Ga mudstone and argillite ~2,200 ft. thick, and are gray to light green in color due to the presence of chlorite minerals, and deposition in a slightly deeper lower oxygen environment
  • The Grinnell Formation is similar to the Appekunny Formation, and is composed of sandstone and argillite ~1740-2590 ft. thick, is rich in hematite, and has a deep brick-red color. This indicates that it was deposited in a shallower oxygen rich environment. The iron content in both formations is roughly equal, and it is the depth of the water and the oxygen levels that give them their distinctive color.
  • The Empire Formation is a transitional formation composed of argillite and siltite ~400-500 ft. thick. It has a combination of features of the Grinnell Formation that lies below it and the Helena Formation that lies above it.
  • The Helena or Siyeh Formation is composed of ~1.1 Ga limestone and dolomite ~2460-3400 ft. thick, and is medium to dark gray in color, but often has an orange buff or honey colored weathering rind.
  • The Snowslip Formation is composed of ~1 Ga red and green argillite and sandstone ~1170- 1600 ft. thick, and is only exposed at high elevations within the park. Like the Shepard Formation much of the rock that involves them has been eroded away.
  • The Shepard Formation is composed of argillite, siltstones, and dolomite, and quartzite ~550-1150 ft. thick. The Snowslip and Shepard Formation make up the base pf the Missoula Group and the top two layers of remaining Belt rock in Glacier National Park.

Chief Mountain

A prominent feature seen in the landscape while travelling eastward in Glacier National Park is Chief Mountain, seen at the right of the photo below. It lies in the northeastern corner of the park on the border between Glacier National Park and the Blackfeet Indian Reservation. At an elevation of 9,080 feet, its unique shape is discernable from miles away, and is regarded as a sacred location to the Blackfeet Indian Nation.


Click here for topographical map of Chief Mountain

Chief Mountain also represents a captivating example of overthrust faulting. This significant structural feature is known as the Lewis Overthrust.

The Lewis Overthrust began ~170 Ma, when Earth’s crustal plates were elevated in a mountain building event called the Sevier Orogeny. Towards the end of this orogeny, a huge slab of rock ~280 miles wide and ~3 miles thick was moved ~50 miles from the west to the east. The huge rock mass containing stronger rocks slid over the softer more easily deformed rock that lie below. The hanging wall moved up and over the footwall, resulting in much older rock being situated over younger rock.

In the photo above, erosional forces have stripped away much of the upper portion of the original Precambrian rock to the left of Chief Mountain, and this isolated remnant of the thrust sheet is called a klippe. Erosion has revealed much of the exposed rocks and structures seen in Glacier National Park today.

The opposite of a klippe is called a fenster, where a portion of footwall is entirely surrounded by hanging wall. It is a tectonic window that has eroded through to lower rock. It should be noted that that this geologic structure does not present itself in the area of Glacier National Park.


In the depiction created above, the fenster can be seen on the right as an eroded opening into the underlying rock.


In this photograph of Chief Mountain taken from the east, the upper plate of overlying Precambrian rocks are more than 1.4 billion years older than the underlying Cretaceous age rocks. Chief Mountain is composed of belt rock from the Altyn and Appekunny formations. The Lewis Overthrust is visible at the base of the Altyn Formation.


Most of the sedimentary rocks from the Belt Supergroup seen in Glacier National Park are Mesoproterzoic in age, and were deposited around 1.45 Ga to 800 Ma. Unlike other areas that have undergone mountain building, these rock formations in the park have preserved many of the characteristic features of sedimentation.

A photo of mud cracks from the Grinnell Formation on the way to Grinnell Glacier. These mud cracks were formed under the same conditions found today in areas exposed to wet, then dry conditions.


A photo of oscillatory ripple marks captured in the Grinnell Formation. These shallow water features are the result of gentle wave action in the fine silty clay.


Another photo of ripple marks in the Grinnell Formation along the way to Grinnell Glacier.


This photo shows mud chip rip-up clasts in the Grinnell Formation, and is indicative of an ancient storm. The high energy currents carried sand and ripped up clay particles which were deposited into the muddy surroundings.



Within the Helena Formation, a fossilized bacterial life called stromatolites can be observed in the sedimentary layers and rock outcrops. They are a cyanobacteria blue-green algae that grew in the relatively warm, shallow waters of the Belt Basin, and their existence can be traced back more than 3.5 Ga, making them one of the oldest lifeforms on this planet. They lived in mat-like colonies and would precipitate limestone on fine sediments onto successive layers. As they continued to grow higher in an effort to reach sunlight, they created their distinctive dome shaped appearance. Stromatolites generate a layer of protective slime that covers their cells, and a new layer can be generated when the old layer gets covered in too much sediment. These dome shaped layers are often referred to as a cabbage heads. Due to the abundant volume of stromatolites during this time, they are responsible for producing much of the oxygen in the Earth’s atmosphere at that time, by consuming carbon dioxide in the water, and releasing oxygen through photosynthesis. The Altyn and Snowslip Formations also contain beds of stromatolites, but the best examples seen in the park are undoubtedly seen in the Helena Formation.


A lesson on the stromatolite layer along Going-to-the-Sun Road, being taught by Professor Callan Bentley. This stromatolite layer is ~3 feet high and 25 feet long.


A closer cross-section view reveals the intricate layers within the matting, and the distinctive dome shape, also called cabbage heads. Note the buff color of the weathered Helena limestone visible above the layer of stromatolites.


This huge exposure of honey-colored stromatolites are found in the Helena formation at Grinnell Glacier. These stromatolites have been ground down over time by the glacier, providing a unique view above this particular colony.


One final picture of the stromatolites with Professor Callan Bentley, which conveys just how expansive this layer at Grinnell Glacier is.

The Purcell Sill

Not all of the distinctive rock layers in Glacier National Park are sedimentary in origin. The dark band that stands out in stark contrast throughout most of the strata within mountains seen in the park is called the Purcell Sill. It is a 130-250 ft. layer of fine grained mafic igneous rock called diorite, which intruded between the layers of lighter colored limestone. The younger 725-775 Ma intruding sill, contact metamorphosed and baked the surrounding 1.1 Ga limestone of the Helena Formation, creating its bleached appearance.


The Purcell Sill shown here at Grinnell Glacier stands out in stark contrast as the black stripe in the middle of the strata.


Another picture of the diorite sill found within the Helena formation at Grinnell Glacier. Note the light colored limestone that has been contact metamorphosed above and below this intrusion.
Looking Glass Road

This outcrop located just outside of Glacier National Park, contains a variety of young sedimentary deposits that range from fine silt beds that crumble in your hand, to lithified beds containing siltite and larger size rocks and cobbles that have been tilted nearly 90 degrees. These layers of strata were deposited as a result of the last ice age during the Pleistocene that ended ~12 ka.

Click here to see the Google Map location


This picture shows the vertical bedding of fine glacial powder, siltstone, and glacial till within the outcrop.

This picture shows the finer grained siltstone, which results from the depositing of glacial powder. There is evidence of dropstones where fragments of rocks drop out of a glacier, and are deposited into the finer grained sedimentary layer below.


This picture shows a close-up of the beds composed of small rocks and cobbles, which are likely the result of glacial till that was transported and deposited by a glacier
Here is a synopsis of what occurred at this outcrop. After the glacier deposited its end moraine, the glacier retreated. As it melted, it deposited fine powdery silt that settled out in a glacial lake. Dropstones were transported, or floated out into the lake within pieces of the glacier, where they dropped into the silt as the glacial pieces melted. Additional gravel and small rocks were deposited in the lake by glacial outwash. The glacier then advanced, acting like a bulldozer, and scraped up all the lake sediments that were deposited previously in its path. The glacier receded a second time, and after melting away, left the vertical beds of sediment made up of the silt, gravel, cobbles, and dropstones seen here along the Looking Glass Road. It is fascinating to see, that the tilting of these beds are so unique, when compared to other structures within the park associated with mountain building.

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