Figure 6 is a lousy graphic. What are the scales of the graph? Is the height or the angle or the pebble-background hourglass shaped areas meaningful? What do the rulings on the horizontal scale mean? What is that “Blast Door” thing meant to suggest?
A failed graph.
That figure has the 60’ scale on it.
It is not a graph but a dimensional comparison.
The gray area is the area the rock is broken down.
Failed comprehension.