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This map shows where oil and gas is found. Is there no oil and gas under the rockie mountains (circled red) or they just haven't found any because it's too difficult to drill through the mountains?
I'm not a geologist so I can't give an answer I'd like, but I do enjoy reading about such things. Your question prompted me to search on my own about it. You should read about the formation of the Western Canada Sedimentary Basin and of course, the formation of the rockies and geologic timelines and forces involved. Wish I could actually provide a short and concise answer, but I just like to think about it and am by no means a scholar. Good luck!
Bonus edit: