IOWA ACADEMY OF SCIENCE 
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The beds of this formation consist chiefly of sands, marls, and 
soft marly sandstones. There are some argillaceous layers pres- 
ent, and frequently some coarse materials. For the most part the 
beds are yellowish or reddish in color. Some of the sand beds are 
slightly cemented by calcareous material. At the base of the sec- 
tion and resting on the beveled edges of the older strata is fre- 
quently a coarse conglomerate bed which is extremely hard and 
firmly cemented. These “cement beds” may not be confined to the 
Santa Fe formation alone and probably are not. 
The Santa Fe formation has a thickness of over 500 feet. Hay- 
den reports a measurement of 1,500 feet. The base of this terrane 
is clearly defined. A marked plane of unconformity exists at the 
bottom of the formation. Wherever this base is observable the 
beds of this terrane are found to rest on the tilted strata of the 
older formations. 
The upper limit of the Santa Fe formation is not as clearly 
marked as the bottom. Over a considerable portion of the area 
which is occupied by the marls the upper layers are mingled with 
the wash-deposits of nearby mountains. In many cases it may be 
difficult to distinguish between the Tertiary and Quarternary beds. 
Inability to separate the formations of the two ages has led John- 
son* to extend Hayden's title so as to cover both sections. The 
facts do not appear to warrant this method of solving a difficult 
problem. Instead of clearing up the obscure points it causes only 
more confusion. 
A critical examination of the Santa Fe marl sections along the 
Rio Grande seems very plainly to show that they terminate with 
the great outpourings of basalt throughout this region. These 
basaltic flows may then be taken as the upper boundary horizon 
of the marls. Since that time the Rio Grande has cut its canyon 
1,500 to 2,000 feet deep, entirely through the surface eruptive 
sheets, the marls beneath, into the indurated formations. This 
has all taken time and is comparable in length to the time con- 
sumed in carrying down the thick wash-deposits which now cover 
the marls and often the lava flows. 
The age of the series is no doubt Miocene, corresponding in the 
main to the Helvetian of Europe. 
The Pecosian series appears to be late Pliocene in age corre- 
sponding to the Astian of Europe. The principal deposits cover the 
Llano Estacado of western Texas and eastern New Mexico along 
the Rio Pecos. 
*Sch. Mines Quart., Vol. XXIV, p. 313, 1893. 
