1 8 A. G. LEONARD 



The shales and sandstones of the Fort Union appear to be very 

 largely of lacustrine origin. The channel sandstones and the con- 

 glomerates which are present in places at the base of the formation 

 were probably deposited by rivers, as were some of the shales and 

 sandstones above, but the greater part of the sediments appears 

 to have been laid down in lakes, many of them of large size and 

 occupying parts of North Dakota, Montana, South Dakota, 

 Wyoming, and a considerable area in Canada. This region is 

 believed to have been a great flat plain occupied by numerous lakes, 

 and over this plain large sluggish rivers took their meandering 

 course. Thus deposits made on the flood plains of rivers, and wind 

 deposits, are doubtless represented in the Fort Union, along with 

 lacustrine deposits, which make up the bulk of the formation. 



Various features characteristic of fluviatile deposits, such as 

 local unconformities and filled channels, are, so far as known, not 

 found in the Fort Union except at the base of the formation, and 

 cross-bedding is of rare occurrence in the sandstones. Were the 

 shales and sandstones of the Fort Union formed chiefly through 

 deposition by rivers the foregoing features should be present, and 

 the fact that except for a little cross-bedding they are not found 

 above the base suggests that the deposits are lacustrine in large 

 part. 



The numerous lignite beds of the Fort Union are evidence that 

 the region was occupied again and again by swamps, many covering 

 hundreds and even thousands of square miles. These were prob- 

 ably formed by the partial filhng of the lakes with sediment brought 

 in by rivers, thus converting them repeatedly into swamps. The 

 coal-forming vegetation growing in these swamps consisted, as 

 determined by Thiessen,^ very largely of coniferous trees, including 

 varieties related to the Sequoia, cypress, juniper, and arbor vitae, 

 together with some firs and spruces. The woody material of these 

 trees, including trunks, stems, and branches, comprises roughly 

 75 to 85 per cent of the whole mass of the lignite. That the 

 vegetation accumulated in some of these swamps for thousands and 

 tens of thousands of years is indicated by the fact that several 



' David White and Reinhardt Thiessen, "The Origin of Coal," Bureau of Mines, 

 Bull. No. j8, 1913, p. 222. 



