376 The American Geologist. Ju"e, iso4. 
THE CLINOPLAINS OF THE RIO GRANDE. 
By C. L. Herrick, Socorro, New Mexico. 
The footing of the mountain ranges facing the Rio Grande 
flood plain presents points of some interest. The valley de- 
posits near Albuquerque have been described by the writer* 
and attention has been called to the probability of a dam across 
the Rio Grande valley at some point south of Albuquerque, 
serving to give rise, temporarily at least, to lacustrine condi- 
tions and to the deposition of the superficial marl called, in 
the papers referred to, "Albuquerque marl . " There is reason 
to think that the basaltic dam at San Acacia is the remnant of 
the blockade refcired to. At any rate, the conditions above 
San Acacia are distinctly different from those below that point, 
and the marl has not been noted below the obstruction, while 
it occurs at the appropriate level to the northward on both 
sides of the river. 
South of the barrier the facies changes. The immediate 
flood plain has a variable width (perhaps a mile on the aver- 
age) and, so far as explored, consists of alternate bands of 
fine loam and gravel ("Rio Grande Gravels'' of the earlier pa- 
pers) the latter being more abundant near the edges of the 
flood plain and probably composed of materials derived from 
deltas of lateral arroyos in an earlier stage. 
This intercalation of gravels also occurs in the flood plain 
deposits and is of great economic importance, for it permits 
the introduction of slotted intake-pipes into the water-bearing 
levels for pumping purposes. Each such gravel bed can be 
converted into a subterranean basin or reservoir by the simple 
process of exhausting the silt mixed with it, so that a short pe- 
riod of pumping produces good water. 
In the valley south of the barrier the slope is about four feet 
to the mile and water in inexhaustable amounts lies within six 
to fifteen feet of the surface. The complete pumping irrigation 
of tliis marvelously fertile land has been delayed by the lack of 
knowledge and the numerous small holdings, largely by native 
people, which prevent the accumulation of a sufficiently large 
• Geology of the Environs of Albuquerque, Amkr. Ghologist, .luly, 1898. 
The Geology of the Albuquerque sheet, Bulletin of Hadley Laboratory, vol. ii, 
1900. 
