THE IRRIGATION AGE. 



977 



on step, for step with the deposition of the Alamosa 

 formation. In other words, the flat alluvial fan of the 

 Rio Grande, with the exception of the gravel veneer, is 

 of sublacustrine deposition, and the fans of the other 

 streams on the west side of the valley are largely of the 

 same origin. The persistency and continuity of the thfn 

 beds of sand and clay making up the Alamosa formation 

 show that this deposition was in the quiet water of the 

 lake at some distance from the mouths of the streams. 

 Near the stream mouths the sedimentation would natur- 

 ally take on the shifting character of delta formation. 

 If sediments of this character occur in the Alamosa 

 formation, they are to be expected in the highest part of 

 the fans and on the border of the artesian basin. 



Though it does not exhibit the close relation of 

 topography and structure shown by the Rio Grande fan, 

 the alluvial slope along the west base of the Sierra Blanca 

 is nevertheless shown, by the occurrence of artesian water 

 well up the slope, to be made up of the alternating clays 

 and sands of the Alamosa formation. Aside from a cer- 

 tain lack of correspondence, as shown by the topographic 

 map, between the topography and the limit of the flowing- 

 well area, there is no apparent reason for distinguishing 

 between the age of the fans and that of deposits of the 

 valley proper. The deposit of gravel that veneers the 

 upper portion of the alluvial fans and slopes is probably 

 more recent than the body of the fans. The section 

 (Fig. 3) and the diagram (Fig. 4) illustrate how the glacial 

 moraines of the Blanca Mountains are superposed upon 

 the crests of the fans, which are thus plainly shown to 

 antedate the ice period. The moraine of Zapato Creek 

 is especially well calculated to bring out this relation, 

 because, as explained on a previous page, almost all the 

 postglacial erosion of Zapato Creek has been limited to 

 the north edge of the fan, leaving the front of the moraine 

 surmounting the alluvial fan, practically as it was disposed 

 by the advancing ice tongue. 



Sand Dunes. 



Incipient dunes occur all over the valley, gathering 

 behind clumps of brush, fence rows, or whatever else 

 offers a wind-break. A particularly sandy area with many 

 small dunes lies south of the Rio Grande between Alamosa 

 and Parma station. Another area lies between Alamosa 

 and Mosca, extending northeastward in the direction of 

 San Luis Lake. The trough of the valley from the rail- 

 way near Washington Springs northward to Dune station 

 is one succession of small brush-crowned hillocks with 

 interspersed bare places. The latter in the wet season 

 become the sites of ponds and lakelets. Each small lake 

 has an embankment on the north and east shores as a 

 result of the lodgment in the vegetation there of sand 

 picked up by the prevailing southwest wind in its clear 

 sweep across the dry bed and shores of the lake. The 

 San Luis Lakes have such embankments 20 feet high and 

 the Russell Lakes have them in proportion. The alluvial 

 slope at the west base of the Sangre de Cristo Range has 

 much drifting sand and many incipient dunes scattered 

 from Zapato Creek as far north as San Isabel Creek. 



The great development of sand dunes, however, is 

 between Medano and Sand creeks, where an area of over 

 40 square miles is a solid expanse of dunes, some of 

 which reach great size, almost small hills. These dunes 

 consist of rather coarse white quartzitic sand with 

 which is mixed a varying proportion of darker and 

 heavier sand, the latter consisting largely of magnetite, 

 with presumably a proportion of those rare earths ordi- 



narily found in black sands. Several attempts have been 

 made at placer mining in the area but are reported as 

 having been given over on account of the difficulty in 

 bringing water to the deposits. Assays are reported to 

 have shown the darker sands to be high in auriferous 

 magnetite. Be that as it may, it seems undeniable tha't 

 the sands of the dune area contain a larger proportion 

 of the heavier material than the sand of the valley. 



Hayden, as noted, classed the dunes as a remnant of 

 the Santa Fe formation. Endlich advanced the theory 

 that the sand came across the valley from the mountains 

 on the southwest, being driven to its present locality by 

 the prevailing southwest winds, and that it was especially 

 collected in the reentrant angle in the mountain front 

 near Mosca Pass as an effect of eddying currents in the 

 winds, caused by the low gaps in the mountains near this 

 point. 



That the sands of the valley are shifting northeast- 

 ward under the influence of the winds is shown by the 

 long sandy area along the western alluvial slope of the 

 Sangre de Cristo Range and by the embankments of sand 

 on the north and east shores of the lakes and ponds. 

 That the accumulations of the great dune area are so 

 derived is open to question. It has been pointed out that 

 the sand of the dune area averages much heavier than 

 that of the valley in general. In accumulations under 

 such wind transportation and sorting, the reverse would 

 occur. It seems much more reasonable to conclude with 

 Hayden that there was here a remnant of the Santa Fe 

 formation, without the protection of interbedded or over- 

 capping lavas, which has been broken down by the winds, 

 and its sands, whipped now forward and now backward, 

 possibly augmented by contributions from the valley, have 

 been built up into the dunes we now see. 

 Alluvial Fans. 



It has already been shown that the alluvial fans and 

 slopes were essentially completed in preglacial time and 

 that they are probably Pliocene or early Pleistocene in 

 age. But over their surface is a covering or veneering 

 of gravel Svhich must have been largely increased in 

 glacial time and since. 



In the description of the topography of the valley 

 reference was made to low bluffs or terraces, not now 

 adjacent to streams, which were explained as old courses 

 of the streams or their distributaries abandoned in the9r 

 wanderings over the alluvial fans. These and the con- 

 figuration of the fan itself seem to have been interpreted 

 by various writers as due to former courses of the Rio 

 Grande. Carpenter alludes to the "ancient course" of the 

 Rio Grande, but does not locate it on the accompanying 

 map. Hinton describes and maps the old course as leav- 

 ing the present river above Del Norte, skirting the foot- 

 hills with a due northeast course to the trough of the 

 valley, which it followed southward, uniting with the 

 present course of the Rio Grande near Hansen Bluff. The 

 course northeast from Del Norte, as outlined on the map, 

 corresponds to the north>edge of the Rio Grande alluvial 

 fan. Doubtless the Rio Grande or some distributary in 

 tijjies past took the course along the north edge of the 

 fan, just as the river at present occupies a course along 

 the south edge of the fan, and just as it must have occu- 

 pied all the intervening territory, wandering backward 

 and forward across it. In this way it must have deposited 

 the gravel and sandy covering of the fan, in the fashion 

 which may be seen after a shower where a gully in a 

 steep slope strikes a more level space and the streamlet 



CONEJOS o * 



MOUNTAINS t- o 



Approximate scale 

 



Santa Fe formation (no lava) 



FIGURE 3. Cross section of San Luis Valley from foot of Blanca Peak to foot of Conejos Range. 



