Luke Limestone at the top of Mount Tyrwitt overheard the chat between Susan Siltstone and Jimmy Shale at Highwood Pass. He feels intimidated by the siltstone-shale combo. “They must have superpowers to fold up like that. I could never do that. I am too old and stiff.”
 
Susan giggles. “But somehow you managed to bend your layers a little bit” she yells up the mountain towards him. Luke glances shyly around. His look settles on the little depression in the ridgeline where the shale layers meet the limestone. Today, it’s mostly covered by snow.
 
“I have nothing to do with the bending of my layers. It was him––Mr. Lewis Thrust fault–– who carried me up to the surface and tilted my layers.” Luke remembers that he used to live a couple kilometers deep down below Earth surface. It was quiet and dark for hundreds of millions of years. But one day, in the Late Cretaceous, everything changed.
 
“I remember rattling and shaking underneath me. It was scary at first, but later exhilarating when I realized that we all moved.” It was Mr. Lewis Thrust who broke through the rock layers from deep underneath Luke and carried everybody who was on top of him towards the northeast and upwards. Some big force from the southwest was eagerly pushing everything to the northeast. Eventually the thrust fault emerged at the surface. The older rocks that were hanging on the back of Mr. Lewis were pushed on top of the younger rocks below him, squeezing them together.
 
Luke Limestone knows that his wight and push caused Susan and Jimmy to fold up. “I am sorry that I squeezed you so much. This must have been painful for you.” Susan looks proudly at her curvy layer. Yes – it was painful when it happened, but it was also 72 million years ago. “Don’t worry – we know it wasn’t your fault. It was Mr. Lewis Thrust.” Susan lowers her voice, suddenly being afraid to wake him up. Mr. Lewis has been resting since the Eocene time. Let’s not wake him up!      

​Susan Siltstone is feeling much better now, after she resolved her dispute with Jimmy Shale. Nobody wants a grumpy neighbor. Since then, many questions came into her mind about her origin. “How did I get so lucky to have all the shales around me, but the limestones up on Mount Tyrwhitt did not?” She tries to remember the times in the Late Jurassic–Early Cretaceous when she formed. Everything was under water. The many small grains composing her siltstone layer all came from the mountainous land in the West. 
 
“I remember the sea was pretty deep, and it was dark at the seabed.” Thinking about this place now, Susan remembers that it was not her first choice––to settle that deep in the sea. That seems like something Jimmy Shale would do. They like to hide in the dark. 
 
“It’s not the dark or deep we like, it is the calm water that made us settle.” Jimmy explains. “We don’t like rushing. We much rather take time to find a quiet place to settle down.” Such calm waters are hard to find, except where the sea is very deep.
 
“They are so sensitive” Susan mumbles. “Only settling where it is really boring. What’s wrong with rushing?” Now Susan remembers that her silty grains first deposited somewhere else––in shallower waters, right at the edge to the deep sea. Shortly after the grains thought they found their place to settle, they all got stirred up again and flushed rapidly down the slope. Like a large avalanche of grains––driven by gravity. This turbiditic rush caused the grains to form a big underwater cloud of grains that quickly deposited on top of the shale––forming Susan’s rock layer. Over time, more shale settled on top of her marking, but occasionally, a big silty cloud rushed down the slope to form another silt layer. Layer by layer building up meters and meters of sediments in the Alberta foreland basin. “The limestones look so different, they must have a different story” Susan wonders looking at the many limestone peaks around Highwood Pass. 

It is +7ºC in Banff. Over the last week, the warm Chinook winds warmed up the Bow Valley and with that Larry Limestone at the east face of Mount Rundle. But Larry doesn’t look happy. “This is too warm and too early. I don’t want the snow to melt!”
Kelly Crinoid is confused. “What do you want? First it is too cold, now it is too warm!” She enjoys every season and the weather that comes with it. Who doesn’t like a chinook in the middle of winter?

But old Larry Limestone feels different about it. He is much more concerned about his face. “These conditions weather me and slowly erode my rock face.” During the warm chinooks the snow melts and water seeps deep into the many cracks in his limestone cliff. But when the temperature drops below 0ºC, mostly during the nights when nobody sees it, the water freezes again and grows bigger. This vicious cycle of water creeping into the tightest corners while thawing, and expanding while freezing and turning into ice again, slowly turns hair-thin cracks into much wider cracks. It always ends dramatically––gravity gets them–– and rocks fall off the cliff. Some are big, others are small.  
 
“I already lost too many pieces to this frost weathering.” Larry looks down at the scree piles that formed in the forest below his cliff. “There is my graveyard of erosion––all the pieces I lost over the years.”

Meanwhile down south at Highwood Pass Jimmy Shale and his friends are upset about Susan Siltstone’s last comment. The shales are not happy that she is always so prominent and gives the impression that it was her idea and her work to deform in a ductile style. “That is a misrepresentation! We, the shales, made all the folding possible!” Jimmy protests. He knows how stiff and rigid Susan really is. “You would have broken apart without our support.” Jimmy is proud of his much softer and malleable nature in comparison to most other rocks. 

Susan scoffs. “Your work? That’s not true at all. I didn’t break because my siltstone layer is thin, just a few centimeters thick. That is the reason I folded easily.” Susan looks west to the top of Mount Tyrwhitt where the old Mississippian limestone layers tower high up into the sky. These guys broke apart during the forceful times of mountain building and are slanting to the southwest since then. “The limestones broke because they are stiff, and their layers are much thicker––tens of meters thick.”  

​“Yes, they are much thick than you and they are 200 million years older than us, but that’s not all. Don’t you see the difference?” Jimmy is flustered about Susan’s ignorance. “There are no shales around the limestone layers.” The whole mountain top is just made up of the stiff and rigid limestone layers. 
​
Now she sees it. It never occurred to Susan before. She slowly wonders if Jimmy has a point here and it was not only her work to make these beautiful folds. “But why did I get lucky and have shales around me while others don’t?” Susan wonders how she got Jimmy around her. 
​

“I can’t hear this guy complaining anymore.” Kelly Crinoid protests. “The warm tropical ocean…..paradise.” Her many sisters giggle as Kelly mimics Larry Limestone. Together they are part of his limestone layer. In fact, the many crinoids’ skeletons make up a good portion of Mount Rundle’s  cliff. 

Kelly believes that her upbringing in the warm ocean of the Mississippian period is really overrated. “I was glad when my animal life ended.” For her, it was only an exhausting start that led to her real destination – being a rock. “So many things were just wrong and felt exhausting.” Her sisters are all nodding in support. “Wait a minute!” old Larry Limestone  is waking up. “What are you talking about?”

Kelly is not surprised about his complain. “For starter: everybody called us sea lilies. But we were no plants!” Larry is surprised. He vaguely remembers that this has been pointed out to him before. “But who cares? Everybody loved you” Larry argues, “they admired your graciously waving petals.” Kelly frowns. “That is exactly the point, these were not petals!” Besides not being recognized as what she was, Kelly also though that being an animal was stressful. “All day long I was waving my arms trying to catch plankton  to eat, then digest it, and then poop it out again.” She still does not see much sense in it and her sisters all smile agreeingly.       
 
Kelly laughs relieved. “Lucky me, it was just for a couple of years. Only a glimpse in comparison to the rest of my 340 million years of happy rock life.” When Kelly the crinoid died, her bones settled on the seafloor and a new chapter started for her. 

While the bitter cold temperatures have a strong hold on grumpy old Larry Limestone at Mount Rundle, he slowly dozes off. His dreams take him back in time––deep time, about 340 million years ago. “Life was so much more fun in the Mississippian time.”  Larry smiles gently following his memories. “This place used to be tropical warm, no mountains, just sunshine and the warm ocean waters.” 

​Back then, Banff was under water. No land as far as you can see (see picture). Only a vast ocean that extended endlessly to the west. British Columbia was not invented, and Alberta was under the shallow waters of the North American continental shelf. “The equator was just south of us.” Larry remembers thinking of the many creatures that lived in the clear ocean waters. “I liked the elegant sea lilies the most. They smoothly moved with the waters and were the prettiest things I have ever seen”
​
And there it was where Larry Limestone formed. At the bottom of the shelf sea. All sorts of life were there, and billions of small creatures were swimming in the waters. Eventually, all their large and small shells and skeletons would gather on the seabed. “It was like a constant snowing of bits and pieces onto my face.” He realized the similarity to the winters now. “But only that these pieces accumulated tens or hundreds of meter thick layers over millions of years.” Over these years, the sunlight disappeared from Larry’s sight as he got buried deeper below the growing rock layers above him. “But it was always nice and warm inside Earth.” He grumbles, still annoyed that he ended up at this freezing rock cliff today.   

Not all rocks are angry about the cold winter weather. At Highwood Pass, Susan Siltstone is happy and excited about winter. This is her prime time to show off her elegant curves. “I love the snow – it accentuates the up and downs of my siltstone layer.” Susan thinks proudly. “We are a real eye catcher.”

Susan refers to her entire neighborhood comprising many thin siltstone layers surrounded by dark-colored soft shale. Like all sedimentary rocks, Susan and her neighbors started out laying flat. “We all settled on the bottom of the big sea that formed here during the Late Jurassic to Early Cretaceoustime about 150–140 million years ago.” Back then, the Rocky Mountains were much further west, over in British Columbia. Back then, the Rocky Mountains were active and wanted to grow not only higher but also wider. The mountains slowly kept pushing everybody to the Northeast.

Susan remembers this time vividly. It was in the Late Cretaceous time. “We had just settled nicely in parallel flat layers and slowly turned into rocks, when the mountain front knocked on our door.” It started with just a gentle push, but later it became quite forceful. The old thick limestone that lived underneath Susan were too strong and stubborn to be squished together. Instead, rocks broke apart at great depths in gigantic horizontal splits. Kilometer thick slabs of limestones were lifted up and shoved northeastward over much younger rocks. “I thought that is the end for all of us.” Susan remembers still shaking a bit. “The old fat limestone layer just did not care about us soft youngsters. It just thrusted over us.” In fact, it was Mr. Lewis Thrust Fault who pushed the 330-million-year-old limestones over Susan’s head. “We may not be strong, but we are clever” Susan frowns at the limestones around her. “We soft rocks sticked together and folded up beautifully like a stack of blankets that gets squeezed.” 
​
Susan is looking over to Mount Tyrwhitt. In between them is Mr. Lewis Thrust who carries those old thick limestones on his back. “These layers look boring” she thinks “they are just a little tilted.” And so, she is satisfied and happy that her neighborhood turned out so much more interesting from the strain of mountain building. Particularly during the winter, when the snow highlights her curvy features.

It's cold this morning! Minus 29ºC. “Brrr – it has been this cold since more than a week now.” Larry complains while shivering. And here at the east face of Mount Rundle it is even colder. At this time of the year the sun is not rising early enough to warm him up. “It is dark and cold. I am so sick of it. Frost bites are all over my face.” Larry grumbles while looking jealously over to his sunny neighbors at the south face of Cascade Mountain. Even the western side of Rundle has it better. That gentler dipping rock face is protected by a thick blanket of snow. That keeps them warm, like the people that walk around in Banff with their winter jackets and snow boots.  

Larry has nothing that protects him. He is exposed to the -42ºC wind chill and nobody seems to care. “I am a rock – we are used to warm and cozy environments.” Larry yells into the cold air trying to let the world know about this injustice. It is one of the misconceptions that rocks are cold. “Who came up with this?” he wonders, “We are warm and most of us are quite hot.” Just a kilometer below the surface it is nice and warm like a sunny Alberta spring day. And another kilometer deeper it is hot like a summer day in the Okanagen Valley. And if you would keep going deeper and deeper into Earth it gets hotter and hotter. 

“You don’t believe me? Well – where do you think the warm waters of the Banff hot springs is coming from?” Larry smirks. This spring water was first rained on the mountain peaks, then traveled deep through the rock layers where it slowly warmed up. Eventually it hit a large zone of broken rocks where the waters all meet to rise again towards the surface. “You see – best evidence of Earth geothermal structure.”

Larry is daydreaming about being inside Earth. It seems to him such a better place than being exposed at the surface and a victim of the weather. “The surface is wearing me out.” he mumbles, “It marks my face with lines and cracks that make me look old.” It is true, Larry Limestone is old, very old. He is in his hundreds of millions of years. But that is a story for another day.