I have recently learned that fossil dating and the belief that geological processes happen slowly is in fact inconstant. Example. Mt St. Helen. The fact is that geologic features form really quick and not over millions of years. Thoughts about this.
Only something like a super flood caused by built-up glacial melt water could create something of that magnitide instantaneously, I think. In this case I think it's safe to say it took millions of years.
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