[dangus]

Well, those rocks probably haven't been in the stream for millions of years. Streams tend to pop up here and there, and rarely persist for more than a few thousand years. The rocks need not be much older than beach glass.

IIt's been too long since I've studied such things -- but I think the reason for the rough edges of the Grand Canyon is smaller-scale wind erosion and weathering (after the large-scale water erosion), which tends to cause chips and chunks to fall off at a time, particularly when the rock in question tends to fracture easily or, like sandstone, is made of a composite of granules and a cementing mineral.

[ngc6656]

IIt's been too long since I've studied such things -- but I think the reason for the rough edges of the Grand Canyon is smaller-scale wind erosion and weathering (after the large-scale water erosion), which tends to cause chips and chunks to fall off at a time, particularly when the rock in question tends to fracture easily or, like sandstone, is made of a composite of granules and a cementing mineral.

There is another possible cause for the introduction of sharp, angular faces to the walls of the Grand Canyon: the same one that creates and perpetuates abrupt waterfalls such as Niagara Falls. When a strata of erosion resistant rock overlays softer materials, the more easily erodible material is carried away, undermining the stronger material above. The undermined stronger rock eventually sheers off because of its weight and falls away to be subjected to continued abrasion and erosion in the river. As this is happening, the Colorado River erodes itself deeper and deeper, leaving the steep, angle faces on the canyon walls above. There is no rushing torrent of water to smooth the high, sharp walls, only the forces of rainfall, wind and the freeze-thaw cycle that are not even remotely as aggressive as the raging river below.