This report provides a historical perspective of age assignment to the various coal fields of the northeastern Colorado Plains and the foothills formations of north-central Colorado. Digital PDF download. B-19D
From the Introduction:
The present paper is the outcome of work originally undertaken in 1912 for the purpose of determining the stratigraphic position of the northeastern Colorado coal beds. As work progressed it seemed desirable to re-examine the problem of the underlying Montana group, especially the upper portion of it.