Aspens and Cytospora?
You may have diagnosed your problem correctly. See this webpage for additional photos. https://gilpin.extension.colostate.edu/programs/natu/canker/
However, while I cannot magnify the photo quite enough to be quite sure, it looks as if you may have oyster shell scale. https://extension.colostate.edu/topic-areas/insects/oystershell-scale-5-513/
You will be able to identify individual scales and "chip" them off of the tree. These are easily treated with an oil spray at the appropriate time. This would be a better more up-beat diagnois than cytospora, as there is little one an do about the fungus....
They definitely can be scraped off so it is probably oystershell scale. Thanks so much for your help.
You bet! You can also consult your local Extension office, which is located in the county administration office on the top floor. https://www.uidaho.edu/extension/county/lemhi