GO DESGEIPTIVE GEOLOGY. 



bluffs, where the country begins to fall off in low, gentle benches of Pliocene 

 strata, overlying the more abrupt cliifs of Miocene which rest directly upon 

 the Laramie sandstones. Along the face of Chalk Bluffs, numerous small 

 streams, ravines, and gullies have worn out good exposures in the soft beds, 

 producing a rugged, uneven surface of clays, marls, and sands. 



From an examination of the country to the west of Chalk Bluffs, it seems 

 evident that, while the Pliocene shore extended close up to within a short dis- 

 tance of the Archaean foundation of the Colorado Range, the waters of the Mio- 

 cene lake, which undoubtedly stood at a much lower level, were hemmed in 

 farther out upon the plains by the gently-rising Mesozoic beds, the beach-line 

 being formed upon the Laramie foi'mation. On the other hand, the Pliocene 

 beds are found resting unconformably upon all the divisions of the Creta- 

 ceous, Jurassic, Ti-iassic, and Carboniferous. 



The Miocene strata are exposed from near Carr's Station on the Den- 

 ver Pacific Railroad, eastward across Owl Creek, the tributaries of Crow 

 Creek, and stretch beyond our eastern boundary. 



Between the Laramie sandstones below and clearly-defined Pliocene 

 beds above, the Miocene presents a development of nearly 300 feet in thick- 

 ness, with an altitude for the basin of about 5,800 feet above sea-level. The 

 line separating the Miocene and Pliocene lake-basins is, however, a difiicult 

 one to determine, as both formations lie horizontally, and consist of beds 

 with much the same lithological composition, with but little evidences of 

 erosion between the two series. So far as the Chalk Bluffs are concerned, 

 the line of junction between the two basins rests almost entirely upon 

 palseontological evidence. 



In general, the Miocene lake strata, in their lithological habit, are char- 

 acterized by a rapidly-changing composition, with a considerable variety in 

 texture, color, and compactness of beds, and are noticeable, in distinction 

 from the Pliocene, for the fineness of sediment and absence of heavy layers 

 of coarse, sandy material. They consist of alternating layers of marls and 

 clays, interstratified with thin beds of fine sand, loosely held together with- 

 out any cementing material. Occasionally, beds will be more compact from 

 the presence of ferruginous layers, which make well-defined horizons. In 



