May 13, 1910] 



SCIENCE 



745 



reality, the southern continuation of that 

 series is found in Wet Mountain Valley and 

 Huerfano Park, occupying a depression be- 

 tween the Front Range and Wet Mountain 

 axis and the Mosquito Range and Sangre de 

 Cristo axis, whereas the San Luis Valley oc- 

 cupies a depression west of the latter axis, and 

 between it and the Sawatch Mountains. 

 Furthermore, the former depression began to 

 take shape much earlier — as far back as the 

 Triassic at least — and has been subject to 

 sedimentation more or less continuously from 

 that time until the Pleistocene, whereas the 

 San Luis Valley shows no formations older 

 than Miocene Tertiary, and is for the most 

 part occupied by late Tertiary or early Qua- 

 ternary sediments. 



In the San Luis Valley there may be dis- 

 tinguished two classes of more or less uncon- 

 solidated gravels, sands, and clays, an older 

 series of conglomerates with intercalated lava 

 flows, known as the Santa Fe formation, after 

 Hayden, and a younger overlying series of 

 blue clays with interstratified sand beds. 



Alamosa Formation. — For the younger upper 

 series of blue clays with interstratified water- 

 bearing sand beds, which occupies the bottom 

 of the valley proper, the name Alamosa for- 

 mation is here proposed, from the town of that 

 name near the center of the valley. 



SECTION OF Hansen's bluff 



Feet 

 Gravelly slope 4.0 



Conglomerate, indurated sandy clay matris . 4.0 



Fine gravel and sand, loose 3.5 



Fine-grained reddish sand 2.5 



Black and red sand 0.5 



I>rab joint clay with a great many white 



indurated nodules 1.5 



Coarse indurated sand and small quartz 



pebbles 4.0 



Buff to light-drab sandy clay 10.5 



Fine and coarse sand in laminae 5.5 



Olive-green sandy joint clay, with shells .... 2.5 



Banded drab sand with clay pockets 1.0 



Fine and coarse pebbly sand in indurated 



laminae 4.5 



Loose black sand 1.5 



Fine banded clayey sand 1.5 



Coarse sand and clay with quartz pebbles . . 2.5 



Debris slope to river 12.0 



The low relief of the valley region renders 

 natural exposures of the Alamosa formation 

 very scarce, the best one being afforded by 

 Hansen's bluff on the east bank of the Rio 

 Grande, nearly east of the Peter Hansen 

 ranch house, and about eight miles southeast 

 of Alamosa. 



Wells in the trough of the valley, at Ala- 

 mosa, east of Mosea, and at Moffat, which 

 penetrate the Alamosa formation to depths of 

 from 1,000 to nearly 1,300 feet, show alterna- 

 tions of blue clay, fine sand with some gravel, 

 and, occasionally in depth, boulders. The 

 water-bearing beds of sand are found at inter- 

 vals of twenty to thirty feet, separated by beds 

 of blue clay. The depth of the first sand 

 yielding a flow at the surface varies with the 

 amount of water drawn from that bed, being 

 greater near the regions of denser population 

 and in the central portion of the valley. The 

 flows from the different water-bearing sands 

 are of different pressures and volumes, de- 

 pending on the depth and thickness of the 

 sand beds. Through these variations it is 

 possible to correlate the sand beds for consid- 

 erable distances in a region where the wells 

 are numerous, and so to establish the continu- 

 ity of the beds. 



The Alamosa formation is readily shown to 

 lie unconformably upon the Santa Fe forma- 

 tion though the contact along the west margin 

 of the valley is everywhere concealed by the 

 long, gravelly, alluvial slope. There is strati- 

 graphic discordance shown by the fact that 

 the lava flows intercalated in the Santa Fe 

 formation dip toward the valley at an in- 

 clination averaging 10°, while the sand beds 

 of the Alamosa formation slope toward 

 the center of the valley with an inclination 

 of less than 1°. In the western and south- 

 ern parts of the valley several isolated hills 

 composed of the Santa Fe formation project 

 upward through the Alamosa formation. The 

 latter formation abuts directly against the 

 Santa Fe formation in the San Luis Hills at 

 the southern end of the valley. These hills 

 and outliers exhibit a much older topograph,y 

 than the younger valley formation. 



The age of the Alamosa formation is 



