Yes, I said "meta" referring to the metamorphic processes that helped change the greywacke we saw before to this new rock. We know that heat and/or pressure must have been present here in order to metamorphose or "change" our greywacke. So what kind of event must have taken place and what would the conditions have been like to bring on such a change? Lets see what the rocks tell us. On observing a fresh hand sample, I see layers in the rock to which the rocks could and do in fact readily break along. I observed Muscovite, which would help explain why this could be happening being that Muscovite is a sheet silicate and has platy cleavage. Muscovite must have come from metamorphosed mud/shale. The precence of mud would indicate calm water deposition..yet another piece of the overall puzzle. The Muscovite is quite abundant along the fractured zones and is in a parallel orientation to the other grains. This had to be the work of an orogenic tectonic event due to the metamorphosis and realignment of minerals.
We know that the Taconian Orogeny took place aprox 450-435 mya, where mountain building, metamorphism and deformation took place as the North American Plate collided with the Taconian Island Arc (Chopawamsic Terrane.) Sediments that were scraped off as the the plates collided formed an accretionary wedge piling up a bunch of mud clasts, sand, and gravel with some pieces of oceanic crust. These are found abundantly throughout the rocks of Great Falls National Park.
As the Teconic Island Arc overode the North American plate, metamorphic processes were at work. Partial melting of the rock led to the separation of the more felsic minerals as they worked their way up through the molten mass. This process was caught in the act as seen in the photo below producing a rock we call migmatite. Here is where igneous rock says hello to meta-graywacky.
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