I’ve also heard that the Grand Canyon was formed very quickly when a debris dam somewhere in what is now MT formed and filled with melting water at the end of the Ice Age. When that dam broke, a gazillion gallons of water rush out and very quickly cut what is now the Grand Canyon. If true, there would be no talus accumulation at the bottom. As least it’s as consistent as the theory posed here.
The missing talus in those canyons is due to advancing glaciers during the ice age. These glaciers pushed the talus ahead of the glacier as it moved through the canyon and eventually left those tumble-rounded boulders hundreds of miles away when the glacier melted at the end of the last ice age.
Yes, you’ll find that talus hundreds of miles away as curiously rounded boulders in the middle of a desert or plain and you’ll wonder how in the world they ever got there.
The Canyon Lake dam, near Canyon City, Texas, overflowed its spillway for several days in 2002, cutting a large gorge beside the dam. People can take tours of this gorge, and the cut made through bedrock. During the tour, the guide tells that studies of this gorge reinforce the thought that the Grand Canyon was cut quickly by flow from a glacial lake giving way. See: Canyon Gorge