STRUCTURE OF THE MANITOU EMBAYMENT 143 



to find the sedimentary rocks on tlie flanlcs of the great Front or Ram- 

 part range of Colorado, in Williams can3^on and elsewhere, resting on 

 a granite surface almost as smooth and flat as a floor and in striking 

 contrast with the existing granite topography of that region. Hayden's 

 illustration, after all, is representative of the entire area and a far better 

 expression of the essential facts than his verbal statement. The actual 

 facts suggest interesting possibilities in the way of erosion and the de- 

 velopment of peneplain surfaces in early times, and suggest also a brief 

 comparison with the facts of other regions to determine whether the 

 phenomena really are as unique as they appear to be on first sight, and 

 an inquir}'- as to the cause or essential conditions of such extraordinarily 

 plane erosion. Having thus indicated the scope of the present paper, I 

 turn to a presentation of the facts of the Manitou area. 



General Structure of the Manitou Embayment 



The Manitou embayment of sedimentary rocks l5'ing in the angle be- 

 tween the granitic rocks of the southern end of the Front range on the 

 north and the similar rocks of the Pikes Peak massif on the south em- 

 braces from below upward, as described by Ha3'den, Cross, and others — 



1. A basal sandstone which is usually 40 to 50 feet thick, white or 

 gray for the lower 10 to 15 feet, and dull red or brown above, only rarely 

 of arkose character, but frequently more or less glauconitic. 



2. This sandstone, which may be referred provisionally to the Cam- 

 brian, becomes calcareous upward, passing into red, cherty limestones, 

 and these into a massive gray limestone having a thickness of several 

 hundred feet. The limestones are throughout more or less magnesian 

 and contain recognizable traces of a Lower Silurian (Ordovician) fauna. 



3. This great Manitou limestone series is overlain without apparent 

 unconformity by the Fountain (Carboniferous) beds, 1,000 to possibly 

 1,500 feet in thickness — a remarkable complex of red and white arkose 

 sandstones, grits, and conglomerates.- 



4. The red sandstone series (Triassic), 1,000 feet or more in thickness. 



5. The white, variegated and gypsiferous Jurassic strata. 



6. The Cretaceous series, beginning with the massive and conspicuous 

 Dakota sandstone. 



As the accompanying map * (plate 14) shows, these formations are cut 

 off on the southwest by the Ute fault with its sandstone dikes, while to 

 the northwest the sedimentaries are seen to lie at low angles directly 

 on the granitic rocks, the highly irregular erosion outline of the embaj'-- 

 ment in this direction, in striking contrast with the straight southwest 



♦Adapted from Hayden : Ann. Rep. U. S. Geol. and Geog. Surve}' of the Territories, 1874, p. 40. 



