CHAPTER Til. 

 POST-LARAMIE AND TERTIARY GEOLOGY. 



SECTION I.— THE ARAPAHOE FORMATION". 



By George ii. Eldridge. 

 INTRODUCTION. 



The Arapahoe 1 is the oldest of the post-Laramie formations of the 

 Denver field. It has only recently been ranked as a formation, notwith- 

 standing the distinct features of its component materials and the nature 

 of its sedimentation. "With the overlying Denver formation it has been 

 regarded by previous observers as part of the Laramie, without discrim- 

 ination either as regards constitution, stratigraphical relations, or forms 

 of life. 



Like many other post-Laramie formations, the Arapahoe occupies the 

 site of an ancient lake, the area of which probably extended considerably 

 beyond the present confines of the formation, at least to the north, north- 

 west, and west. Along the northern and northwestern edges the formation 

 now appears only as a thin horizontal sheet, or in scattered outliers upon 

 the uneven surface of the underlying Laramie. Along the western out- 

 crop, win-re the strata are highly inclined and confined between under- 

 lying and overlying terrain's, the formation is 600 to 800 feet thick, the 

 size of material and the position relative to the mountains indicating still 

 a distance of at least several miles from the original shore-line. To the 

 south and east the younger formations have not yet been removed from 

 the Arapahoe, and the extent and nature of its shore-lines can not be 



1 The name Willow Creek was at first employed to designate this formation, but was changed to 

 Arapahoe iu a footnote to the original paper. Proc. Colo. Sci. Soc., Vol. Ill, p. 86, Denver, 1889. 



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