The Mesoproterozoic Belt Supergroup of Glacier National Park offers opportunities for both students and professionals to refine observational skills and make interpretations based on a combination of geologic first principles and outcrop-scale data. Over the last 10 years, we have brought students and colleagues of varying experience to GNP. Not only do these rocks serve as a superb introduction to Proterozoic sedimentary facies, but many of their features challenge even experienced geologists to step away from conventional uniformitarian models and to reconsider how geologic hypotheses are formulated.

The basal Altyn Fm contains an unusual mixed sand-carbonate lithology that contains elongate ridges of stromatolites, the axes of which migrate sinusoidally with time. Even in the admittedly peculiar repertoire of Proterozoic stromatolites, these forms are unusual. The overlying Appekunny Fm has its own challenges; its depositional environment has been interpreted as either deep marine or supratidal, and it contains enigmatic sedimentary structures (“strings of beads”) that challenge observers to imagine ways in which they might have formed. By contrast, shales of the Grinnell Fm contain a wide variety of ripples, mudcracks, and raindrop impressions, along with coarse sand and rip-up breccias, that are easily interpreted as shallow-water to supratidal facies, but its ~600 m thickness requires consideration of how the necessary accommodation space was generated. Finally, the Helena Fm contains an enigmatic Proterozoic carbonate fabric (“molar tooth”) whose formation demands release of preconceived notions regarding carbonate formation. Excellent outcrop and rough-hewn blocks along Going-to-the-Sun Road permit observers 3-D views from which to work from first principles in determining the origins of these features.

Sedimentary rocks of Glacier National Park provide a unique opportunity within a restricted geographic area for geologists to hone observational skills and practice posing, testing, and defending geological hypotheses. The co-occurrence of non-actualistic sedimentary rocks in a relatively accessible setting means that generations of curious geologists can revisit these outcrops and contribute to our as-yet incomplete understanding of the Belt Supergroup.