hay. I SURFACE GEOLOGY OF THE GREAT PLAINS. 445 



age having made more impression as we proceed east, the narrowing area of the 

 uplands lias less of the Plains marl, and east of the ninety-eighth meridian it is only 

 recognized in small isolated areas. (Joining east it becomes more and more loess- 

 like and in Nehraska and northern Kansas it merges into that formation, which is 

 typically developed in the bluffs of the Missouri River. In eastern Colorado and 

 western Nebraska and Kansas the Plains marl is usually underlaid by well-defined 

 Tertiary formations. About the one hundred and fourth meridian on both sides of 

 the Platte River these are the White River strata principally, which may be named, 

 from their lithologic character, the Mortar beds. Coming east and south these thin 

 out and give place to the Loup Fork, into which they merge. This last formation 

 has within it beds of conglomerate, 1 > 1 1 1 whether as fresh-water lime beds, mortar 

 beds, or conglomerates, the Loup Fork is everywhere a water holder; and this is 

 true also of the thicker White Rivet beds Stretching toward Wyoming and north- 

 west Nebraska. The more impervious Plains marl lets the meteoric water percolate 

 slowly through it, and the looser arenaceous or gravelly texture of the White Rivex 

 or the Loup Fork beds holds tlie water till it is reached by wells from above, or 

 escapes in springs in ravines where erosion has cut sutliciently deep. The ever- 

 present arenaceous character of these Tertiary beds, whether as mortar beds or con- 

 glomerates, and their consequent water-bearing capacity have suggested the term 

 Tertiary grit as a designation showing their relation to economic geology. These 

 two formations, the Plains marl and Tertiary grit, make the essential features of the 

 mid-plains geology. The mammalian and reptilian (turtle) fossils of the grit have 

 been described by Marsh and Cope. Some floral remains and fresh- water univalves 

 have also been found. Underneath the marl and grit in all the mid-plain region lie 

 Cretaceous formations. These are all more impervious than the Tertiary grit and 

 so help to make the water-bearing character of that formation more decided. 



