Man, I need to slow down a little but I have an interesting story/experience to tell.
A while back I worked with a company that installed gas sensing systems in coal mines. The most common coal mine left huge pillars of coal 75' x 75' to hold up the roof. The coal was mined between these "stumps", creating a gridwork of tunnels about 15' across and ranging from 8 to 10 feet high. The coal that was mined was probably less than a third of what was there.
The Monterey mine in Illonois, operated by Exxon, used this technique of mining along with another method called "continuous mining". This system required that the land above is flat not like the mines in West Virginia that followed the seams of coal through the folds and contours of the mountainous regions in that part of the state. What they did was mined entire panels of coal 10,000 feet long and 7500 feet wide.
The equipment was brought in after mining in the normal fashion around the border of the proposed panel of coal to be mined. On a chart it looked like a large box with a border around it. The equipment was setup at the far end of the box. Huge clamshell rams each 3 feet wide were used to hold up the roof and were placed side by side for the width of the cut, one long row of rams standing together. From the side these rams looked like a large capital "C" whose height was varied by two 10" hydraulic cylinders. The coal was mined from in front of the rams the entire width of 7500 feet then every other ram was released, collaped, moved forward, again by hydraulics, then expanded again to hold up the roof while the other rams did the same thing. Once all was in place the mining continued. This was really a sight to see. The roof behind these rams collapsed, falling as much as 10 feet. I left footprints where no man will ever walk again.
But there was a problem. The mine had to buy the surface ground which was farmland. In that particular area the main crop was corn. When the coal was extracted 300 feet down the cave-in caused the the corn to slant at as much as 45 degrees or more. It was an odd thing to see to say the least. The ground fell in much like the crust of a pie whose filling had been removed. And there were patterns like snaking rows where the corn grew funny.
Years later I moved to Salt Lake City. During that time I needed to book a round trip flight. At 35,000 feet above the Rockies I looked down and I thought, "I've seen this somewhere before." Minus the corn. I just stared at the scene below. There was a mini-landscape in Illinois that looked just like the landscape I was seeing.
Also at that mini-landscape in Illinios there were also large areas that didn't cave-in right away as the coal was removed causing areas of flat ground surrounded by the shattered landscape.
Now, back to the plane..
On the return trip I got a view of the Rockies and the Salt Lake valley together. Again there was a mini-landscape like the one back in Illinios though this one was on a much grander scale of course. The Salt Lake Valley is a flat expanse of land with the Rockies on one side and the Oquirrh (pronounced "oaker") mountains on the other.
Living here in Salt Lake I can see from my backyard huge slabs of rock that make up as much as half of a mountain almost standing on end. From this viewpoint it looks very much like there was an enormous cave-in deep under those slabs.
Could it be?
Also looking at the Appalachian mountains in Pennsylvania and Virginia I see folds and not the sudden breaks as seen in the Rockies. The Rockies are of a much harder rock, mostly granite whereas the Appalachians are mostly of a much softer rock like sandstone, limestone or rock formed from clays. Removing water from under a softer rock would produce "waves" like that of the surface of hot pudding when the underneath is removed. Yeah, I played with my pudding as a kid to enjoy every bit of fun it had to offer.
Especially chocolate. <drools>