Monterrey lies in a plain at the foot of the Eastern Sierra Madre
Mountains which constitute the eastern margin of the Mexican Cordilleran
Plateau, and is surrounded by the magnificent mountains of that group,
among the most notable of which are the beautiful Mitra and Silla
Mountains. In the neighborhood of Monterrey these mountains attain
heights of from 2,000 to 2,400 m., and are noted for their broken and
jagged sky-lines. The leading geological characteristics of the district
are the uplifted limestones of the older cretaceous age which form the
main mass of the mountains.
Primarily, the mountains are compressional folds which, in the Sierra
Madre, near Monterrey, are close and vertically compressed.[2] The
drainage areas of the Santa Catarina River, which flows through
Monterrey, and of the Estanzuela and Silla Rivers, its tributaries, are
of limestone and shale; originally the shales were above the limestone,
but the convulsion which formed the Sierra Madre as an anticlinal fold,
left the originally horizontal strata standing nearly upright, and
subsequent erosion in the upper part of the anticline has exposed nearly
vertical strata in many places. The limestone being hard and resisting
erosion, there is generally, along the line of contact, an abrupt drop
vertically on the face of the limestone to the shale below. In many
places this abrupt drop is broken by a limestone talus, but the line of
contact can generally be traced. Mining operations in these mountains
have revealed the presence of large caves at a considerable elevation,
many of which contain large reservoirs of water, delivered to them
through numerous faults. The river valleys are formed of masses of
limestone conglomerate and coarse gravels, re-cemented in many cases by
the lime deposits of the flowing waters. One of the chief
characteristics of the subsoil of Monterrey itself is a local rock
called "sillar," which is a superficial deposit of carbonate of lime
from the evaporated waters. In some places the "sillar" is largely mixed
with a conglomerate called "tepetate," or "impure sillar."
[2] _Transactions_, Am. Inst. Min. Engrs., Vol. XXXII (1902), pp.
163-178.
[Illustration: PLATE II.--GENERAL PLAN OF THE WATER SUPPLY AND DRAINAGE
WORKS FOR MONTERREY, N. L., MEXICO.]
Topographically, the region around Monterrey is distinguished by the
drainage area of the River Santa Catarina, which rises in the Sierra
Madre near the Laguna de Sanchez, at an elevation of 1,850 m., as shown
on Plate II. From this Laguna it follows a tortuous course between
precipitous mountains through the Boca of Santa Catarina to Monterrey,
for a distance of 90 km., eventually finding its way to the San Juan
River, a tributary of the Rio Grande. Throughout its course it
disappears, flows underground, and again appears; and, except in flood
time, it has a subsurface flow for a distance of 16 km. above the city.
In the Canon of Santa Catarina it appears at the surface, having a
normal flow of about 1,415 liters (50 cu. ft.) per sec., and its waters
at that point are divided into two parts and carried into irrigation
canals. The drainage area of the river above Monterrey is 1,410 sq. km.,
and its bed at Monterrey is between 518 and 545 m. above sea level.
Southward from Monterrey the country rises along the valley of the Silla
for a distance of 19 km., where the Silla is separated from the San Juan
by a low divide, the former flowing northward to Monterrey and the
latter southeastward toward Allende. The Silla Valley is bounded on the
east and west by the steep ranges of the Silla and Sierra Madre
Mountains. The floor of this valley is gently rolling, but is cut by
many arroyos which carry little or no water during the greater part of
the year. The chief feeder of the Silla River is the Estanzuela, a
stream which derives its waters from several springs coming to the
surface near the line of contact between the limestone and the shale, at
elevations of about 800 and 900 m.[3] above datum. The water-shed of
this stream is rich with abundant vegetation due to the precipitation
being greater than on the Santa Catarina water-shed. To the south of the
divide the country is well wooded, and El Porvenir, 35 km. from
Monterrey, is the garden spot of the State of Nuevo Leon. Here the
rainfall is much greater than at any other point near Monterrey, and
there are many streams which are used for irrigation purposes. Monterrey
is built on a plain, chiefly on the north side of the Santa Catarina
River. This plain has a general fall toward the northeast, and beyond
the city it slopes gently northward for several miles toward the Topo
Grande River, and then southeastward to join the great coastal plain of
the Gulf of Mexico. The general elevation of the city lies between the
519- and 550-m. contours. The Plaza Zaragoza, in the center of the city,
is 533.90 m. above sea level; the elevation of the highest part of the
city, at the western boundary, is 550.05 m., and of the lowest part, at
the northeastern boundary, 518.0 m. above sea level.
[3] Throughout this paper datum refers to the height in meters above the
mean sea level of the Gulf of Mexico at the Port of Tampico.
[Illustration: PLATE III, FIG. 1.--GENERAL VIEW OF LINE, ESTANZUELA
AQUEDUCT.]