[blam]

"I'm not a subscriber, so I can't view the whole article, but I'm curious. How was the Ouachita-Appalachian barrier breached? An earthquake?"

I'm not either. I'll venture a guess though. An Ice Age lake breached theOuachita Mountains?

[Ben Ficklin]

Wiki's statement that the remaining Ouachitas in AR and OK are remnents of the original mountains is not exactly accurate.

The quartz deposits that Wiki mentions is what has preserved what's left of the Ouachitas. Each mountain/hill has a horizonal bed of quartz located towards the top that is exposed around the entire perimeter, protecting the mountain/hill from erosion.

The original mountains that did not recieve the mineralized water and resulting quartz bed are long gone.

The Ouachita Natl Forest website has a good explanation of the geology.