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.