MiRVLXE.] (GEOLOGY ROCKS EAST OF FRONT RANGE TRIAS. 97 



makes it convenient to place the plane of separation between these two 

 lower series just at this horizon, there beiog no fossils known in the 

 beds to afford any paleontological grounds for a separation at any par- 

 ticular plane. 



Gentle folding near the north has in places left the red beds forming 

 several ridges, but between the Saint Vrain and Left-Hand Creeks 

 they run into one, and the whole series occurs as a simple upturned 

 ridge dipping away from the mountains at an angle of about 35°, and 

 as a simple fold it continues on southward, only varying much in dip, 

 being in places even turned slightly past the vertical. The genei-al 

 characters, as given at the Little Thom]3son, also slowly change in 

 approaching Left-Hand Creek. The series thickens slightly ; the top is 

 a thickening band of yellow 5 the shales have diminished in amount, 

 and the lower part has less conglomerate and yellow, while the general 

 softness is such that the ridges formed are not prominent. These litho- 

 logical features remain much the same to Bowlder City. Here a sudden 

 change in the hardness of the red beds occurs; they become massive 

 nearly throughout, and all the lower 800 feet is of a deep dark red. 

 The hardness is so increased in these lower beds that they rise in a high^ 

 ragged ridge reaching over 2,000 feet above the plains, and even con- 

 siderably above the granite buttresses against which they rest. This 

 ridge of hard sandstone reaches south far enough to be cut by both 

 Bear Canon and the South Bowlder, the caiion of the latter being quite 

 profound, while between the two canons the ridge attains its highest 

 point in Bowlder Peak, one of the stations of the primary triangulation. 

 The latter is, in reality, on a second or inner ridge of sandstone, which 

 has been faulted down from the outer one, as will be shown more in 

 detail later. (See Plate II, section 11.) These caiious give excellent sec- 

 tions of these lower rocks, but this exceptional hardness at this point 

 does not make them tyi>ical. In places, however, the origin of the lower 

 sediments is well shown, being often composed of a rather coarse aggre- 

 gate of quartz and feldspar, with frequent pebbles of adjacent quartzitic 

 and granite rocks. 



Lying over this lower 800 feet of red beds, which thus attain an alti- 

 tude higher than any other sedimentary rocks east of the Front range, are 

 about 300 feet of free, clean, gritty, siliceous sandstone, very light yellow 

 in color, inclined to massive in bedding, but cleaving with a fine plane 

 surface. They do not reach very far up on the higher ridge of their 

 harder underlying red companions. 



Near Coal Creek the red beds have again become softer, and are 

 eroded away till barely exposed. At Ealston Creek they again rise at 

 a low angle to a considerable height, but only to again fall to a valley 

 trough a few miles north of Golden City, where they begin to thin out 

 in a remarkable manner, and at that city scarcely reach a thickness of 

 400 feet. Their relations, however, are here not wholly clear, as they are 

 quite soft and have been eroded nearly away, the valley trough in which 

 they should outcrop being well sod-covered. They dip from between 

 40° and 50° near the granites, which form the very abruptly rising 

 western border of the valley, but near their top are thrown somewhat 

 ^jeyond the vertical. Section 4, plate I, is scaled from a section given 

 uie by Mr. Berthoud, of Golden City. 



' South of Golden City the red beds again rapidly thicken, and con- 

 tinue to thicken all the way southward to where the South Platte de- 

 bouches from the mountains, the southern limit of the district, where 

 they attain a thickness of over 1,000 ieet. Throughout this southern 

 region, though still massive in general character, they are yet quite 

 7 G s 



