976 



THE IRRIGATION AGE. 



Geology and Water Resources of the San 

 Luis Valley, Colorado* 



By C. E. Siebenthal 



Stretching northeastward across the valley from An- 

 tonito to Fort Garland is a series of basaltic hills, flat 

 topped and higher west of the Rio Grande, lower and 

 more rounded east of the river, degenerating into a lava- 

 capped mesa, the north border of which forms an escarp- 

 ment along the south side of Trinchera Creek. At a 

 point 5 miles southwest of Fort Garland this escarpment 

 swings south, joining the San Pedro Mesa at San Luis. 

 These hills and mesas form the southeast limit of the 

 artesian basin, the sand and clay beds of which abut 

 against the older formation, numerous springs coming 

 up along the contact, particularly in the lower course of 

 Conejos River. This contact in all probability marks a 

 fault scarp, the older strata forming the west ranges and 

 the floor of the valley being deeply downthrown to the 

 northwest in the formation of the depression in which 

 the sands and clays of the artesian system were deposited. 



In the San Luis Valley there may be distinguished 

 two classes of more or less unconsolidated gravels, sands, 

 and older series of conglomerates with intercalated lava 

 flows, and a younger overlying series of blue clays with 

 interstratified sand beds. 



The older conglomeratic series makes up the small 

 isolated mesas and the higher foothills about Fort Gar- 

 land and southward along the western base of the Culebra 

 Range, as well as the basalt-covered San Pedro Mesa and 

 its northward continuation which has been mentioned in 

 the discussion of the San Luis Hills. Hayden describes 

 these mesas and refers them to the Santa Fe formation. 

 A thin section of some of the consolidated sands from 

 beneath the lava in the north end of the San Pedro, Mesa 

 near San Luis shows, microscopically, a prominent cal- 

 careous cement. The sand itself is in part volcanic debris 

 and in part of aqueous origin. The age of the Santa Fe 

 formation has been shown by Cope from w,ell-known and 

 characteristic vertebrate remains to be Miocene. The 

 mesas east and south of Fort Garland are capped by a 

 flow of basalt, while sheets of the same rock are inter- 

 bedded with the gravels and sands of which the mesas are 

 composed. 



A series of deposits northwest of Fort Garland, the 

 "compact drift" of Endlich, deserves notice. A great 

 alluvial fan that formed on the left fork of Ute Creek 

 has been trenched by the creek and shows a succession 

 of terraces. Northwest of Fort Garland a mesa approxi- 

 mately 150 feet high is apparently A portion of this fan, 

 which, with others formed by the other streams that 

 converge at Fort Garland, filled up the angle of the 

 valley between the Blanca group on the northwest and 

 the basalt-capped mesa southeast of Garland. The mesa 

 is covered with a sheet of gravel ranging from pebbles 

 up to bowlders 12 inches in diameter, and is made up of 

 fine buff sand or unindurated sandstone, with here and 

 there a gravel layer consisting of various kinds of crystal- 

 line rocks, the whole dipping 10 to 12E. In the next 

 mesa due west, up the draw, the dip of the solt sandstone 

 is somewhat steeper, ranging from 12 to 22 in a direc- 

 tion north of east. Some of the sandstone beds are quite 

 indurated. No fossils are to be observed. The dip con- 

 tinues on across the mesa for a mile, but shifts to the 

 south. Toward the west edge of the mesa the high points 

 are composed of igneous rocks projecting through the 

 sands. Within 200 yards of the igneous rock the sand- 

 stone becomes more and more bowldery, until in the 

 immediate neighborhood of the igneous rock it is com- 

 posed almost exclusively of bowlders and all stratification 

 is lost. The igneous rock has not been intruded into the 

 sandstone, for the contact is everywhere such as would 

 result from the deposition of the sandstone upon a surface 

 that had been subject to subaerial weathering. Distin- 

 guishing these beds from Tertiary lake beds under the 



'Abstract from Water Supply paper 240 of the United States 

 Geological Survey. 



appellation "compact drift," Endlich correlated them with 

 the valley glacial drift of the Sangre de Cristo Range, 

 though believing them to belong to an early stage of the 

 glaciation. Hayden referred them to the Santa Fe forma- 

 tion, and the present writer coincides in that correlation. 



Another occurrence of the older material is in the 

 great sand dunes west of Mosca Pass. Endlich regarded 

 the dunes as of recent origin and derived from the drifting 

 sands of the valley, but Hayden referred them to the 

 Santa Fe formation. 



No fossil remains are known to have been found in 

 the Santa Fe formation within the limits of the San Luis 

 Valley, its age determination depending upon localities 

 farther south in the Rio Grande valley. 



Though it is impossible to correlate well sections that 

 are any great distance apart, or to trace any stratum of 

 clay or sand from point to point, nevertheless the ex- 

 perience of drillers is that the "first flow" (and by infer- 

 ence, the successive lower flows) in any neighborhood is 

 found at very near the same horizon or depth, gradually 

 increasing in depth toward the center of the valley. For 

 instance, the first flow in the region of Monte Vista is 

 at about 100 feet, near Parma 175 feet, and at Alamosa 

 about 250 feet. Between Center and Hooper the first 

 blue clay is struck at about 80 feet along the entire dis- 

 tance. At Center the first flow, a very small one, is found 

 at 155 feet, at Garnett the first flow is at about 175 feet, 

 and at Hooper and Mosca at about 200 feet. That the 

 uppermost water-bearing bed is covered by a stratum of 

 clay persisting from the center 'of the valley to the receiv- 

 ing area in the gravelly alluvial slope at the edge of the 

 valley is likewise indicated by the fact that the water is 

 not alkaline. If the bed came to the surface anywhere 

 inside the irrigated area of the valley, it would necessarily 

 absorb more or less alkali from the irrigating waters. 

 Even in the Mosca-Hooper district, where the tinted 

 deeper waters are highly alkaline, the uppermost flow is 

 clear and potable. 



The cross section in figure 3, though largely ideal, is 

 based upon all the data available. A fact that comes 

 out in the construction of this section is that wells near 

 the center of the basin show heavy clay beds and fewer 

 aquifers; that wells nearer the margin of the valley, but 

 not too close to it, show thinner clay beds and more 

 aquifers; whereas wells at the limit of the flowing-well 

 area, or outside that line, show but a few and thin beds 

 of clay, with sand largely predominating. This is but 

 an expression of the physical limitation of sedimentation. 

 Sand, being coarser and heavier than clay, would be 

 expected to constitute the bulk of the deposits near shore 

 that is to say, near the edge of the valley while the 

 finely divided clays would make up the greater volume of 

 the formation in the center of the valley. Beds of sand, 

 thinning toward the center of the valley, pinch out before 

 reaching that region, and beds of clay thin out before* 

 reaching the margin of the basin. Thus the region where 

 both clay and sand beds exist in greatest number is a 

 band of indefinite width which circles the basin some 

 distance inside the margin of the flowing-well area. These 

 features are shown in the diagram (Fig. 4). If it be 

 argued that the lower aquifers, having the heaviest pres- 

 sure, must be confined by clay beds which extent farther 

 up the alluvial slope in the region of intake, it should 

 be remembered that the pressure where the wells are 

 numerous is dependent largely on their number. The 

 deeper the aquifers the fewer are the wells that reach 

 them. Of two aquifers drawing from a common bed of 

 sand in the marginal region, and divided by a clay septum 

 only part way back from the wells, and hence theoretically 

 with identical pressures, the lower one will nevertheless 

 exhibit the higher pressure if the upper one is more drawn 

 upon by wells. 



Attention has been called to the great development 

 of alluvial fans and slopes about the sides of the valley. 

 The great Rio Grande fan occupies a fourth or more of 

 the whole extent of the valley bottom. From the figures 

 which have been given of the depth to the first flow at 

 various points on this fan it is evident that the configura- 

 tion of the uppermost aquifer agrees closely with the 

 surface of the fan, at least within the limits of the artesian 

 basin. The formation of the fan up to the horizon of 

 the upper water-bearing sand must therefore have gone 



