The Anatomy of Mountain Ranges

Chris is back!! 

Today we answer the simple question - Why do we see a predictable geologic and topographic progression as we drive from flat plains into mountains? 

We use examples from Michigan to Tennessee, the Canadian Rockies, Glacier, the Bighorns, Colorado, and the Appalachians to walk through a common sequence: we start on broad areas of mostly flat-lying sedimentary rocks (sandstones, shales, limestones) deposited in shallow seas, rivers, intertidal settings, and deserts; as we approach the range, we cross subtle, long-wavelength, low-amplitude folds that are often hard to notice without measurements; then we enter the fold-and-thrust belt where anticlines, synclines, and large thrust faults stack sedimentary packages and create dramatic ridges, valleys, and cliff faces (thin-skinned deformation). 

We explain how the growing mountain load flexes the plate to form a foreland basin that fills with sediment eroded off the range, typically thickening and coarsening toward the mountains. Farther inboard, we describe how erosion and unloading help exhume deep, high-grade metamorphic “roots” in metamorphic core complexes (gneiss, schist, and other intensely metamorphosed rocks), and how overthickened crust can later relax and extend, aiding exhumation. 

We also discuss how some mountain belts preserve suture-related features like ophiolite complexes, while others show subduction-related batholiths (e.g., Sierra Nevada, Idaho Batholith), and we note modern analogs such as the Persian Gulf foreland basin.

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