So from what I've been taught, greywacke are poorly sorted sandstones with dark tones because of the higher amount of matrix. The poor sorting happens because of the rapid deposition of turbidity currents near the continental slopes. In thin sections under the microscope the framework grains show angular shapes and are surrounded by the matrix. (You can see examples of thin sections in the linked website)

But looking at the thin sections, there's something that seems kinda counterintuitive to me. Why are the framework grains angular-subangular? These sediments are deposited in the sea, so they should have been rounded out either by the long travel inside a river from the source site, or by the constant reworking of the sand by the waves near the coast before arriving at the continental slope.

Even in the case that the grains were actually from a nearby outcrop and they didn't have time to become rounder, I imagine there would still be at least a portion of rounded crystals mixed in the framework simply…