FORMATIONS OF THE FRONT RANGE 419 



In the steep dipping beds west of Boulder a thin bed of similar lime- 

 stone occurs 70 feet above the top sandstone (Tensleep) of the Lower 

 Wyoming series, from which it is separated by the usual series of bright 

 red sand}' shales. It is overlain by gypsiferous red beds. 



In the monograph on the Denver region the Red beds are described 

 in considerable detail in their extension from the vicinity of Boulder to 

 south of Deer creek under the terms upper Wyoming and lower Wyo- 

 ming, the latter lying directly on an irregular surface of granite, as in 

 the region northward. The thickness of the lower Wyoming is found 

 to be exceedingly variable, ranging from 200 to 2,400 feet, while the 

 upper Wyoming varies from 400 to 600 feet, excepting for a short dis- 

 tance near Boulder and Golden, where it thins out entirely. The vari- 

 ations in thickness of the lower Wyoming are due mainly to the 

 inequalities of the granite floor on which it lies, the thicker portions 

 lying in deep basins and the thinner portions being where there is over- 

 lap of the higher beds on the slopes of these basins. At the granite con- 

 tacts at all horizons there are usually coarse cross-bedded sandstones 

 and conglomerates. 



Succeeding the basal deposit there is a series of massive, cross-bedded 

 sandstones and grits, with small local bodies of sandy shale. The normal 

 thickness of this series is about 1,200 feet, and its color varies from pre- 

 vailing red to gray, the finer grained beds being, as a rule, the most 

 highly colored. This bed is shown in foreground of figure 1, plate 32. 

 Toward the upper part of the lower Wyoming there is a transitional zone 

 of lighter red and more quartzose sandstones, which is terminated by a 

 characteristic member of heavily bedded white sandstone from 200 to 

 400 feet thick, designated the "Cream}' - sandstone." This member usually 

 forms a well marked ridge 50 to 100 feet high along the middle of the 

 valley of Red beds. Two small bands of dark brown quartzose lime- 

 stone from 2 to 8 feet thick are present, the lower bed being near the 

 base. 



The upper Wyoming division consists mainly of red, sandy shales, 

 such as are typical of the " Red beds " of the region north. " Limestone 

 layers occur within 75 feet of the base, usually 3 or 4 beds of it from 6 to 

 18 inches thick in the lower 15 feet, and 50 feet higher up a bed 5 feet 

 thick, with red, sandy shale intervening. The upper bed is overlain by 

 a bed of thin, wafer-like layers of white limestone and red mud, in all 

 5 or 10 feet. The principal members above the limestones are red, sandy 

 shales, with the amount of sand increasing gradually in the higher beds." 

 Thin sandstone layers occur, and occasionally a red sandstone bed be- 

 comes prominent. " Higher up, or from 150 to 200 feet below the top 

 of the formation, the strata become more clayey, and present a variety 



