Susan Siltstone’s curves at Highwood Pass

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.” 
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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.