Reconnoissance soil survey of South Texas Page: 46 of 115
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44 FIELD OPERATIONS OF THE BUREAU OF SOILS, 1909.
matter, and every opportunity to increase this by the use of green
manures should be taken advantage of.
The subsoil has very much the same texture as the surface soil,
consisting of a gray to almost white loam containing a very high
percentage of lime. Its very light color is largely due to this large
amount of lime present.
The areas of loam, which are undifferentiated in the map from the
fine sandy loam in the more western extension of the latter type,
although they occur more often in the lower part of the valleys,
differ somewhat from the main body of the type in having a heavier,
less calcareous subsoil. This sometimes has also a slightly pinkish
tinge and has probably been influenced more by recent stream action.
One striking feature which seems to be confined to areas occupied
by this type is the occasional presence of cracks, sometimes as much
as 3 feet across, several hundred yards in length, and extending to
unknown depths, as the dirt from above has fallen in and filled
them to within 3 or 4 feet of the surface. A crack may divide and
extend in two directions, or it may be crossed by a similar one running
in another direction. Their cause could not be ascertained,
but they are probably due to some readjustment resulting from
heavy rains or the addition of irrigation water.
Origin.-The Brennan loam is a sedimentary soil,' being derived
from calcareous deposits of sand, silt, and clay. The material consists
of a mixture of the sands which make up the Brennan fine
sandy loam with the gray silt from the Rio Grande. This process did
not take place in Recent Quaternary, but rather in Pleistocene time,
and the deposits may be of the same age as the higher terrace formations
of the upper Rio Grande, which give rise to the Laredo silt
loam. It is very distinct from the soil of the more recent delta
Location.-The principal body of the Brennan loam is found in
the vicinity of McAllen and San Juan, from where it extends north to
Chapin, and east from this point as a narrow strip almost to Combes,
and also south to Mercedes and thence westward to Donna. Another
small body extends west from a point south of McAllen, including
Mission, Mamie, and Chihuahua. The latter areas, however, embrace
almost as much of the fine sandy loam as the loam, but on the whole
are heavier than the latter type as found farther north. Numerous
small, irregular areas, which could not be differentiated in the map
either because of their small extent or the almost impenetrable
character of the country, are scattered through the large body of
Brennan fine sandy loam north of these points. About 12 miles north
of Closner numerous small areas from 50 to 100 acres in extent dot
the country and are especially noticeable on account of the heavier
timber growth which they support. In Webb and Zapata counties
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Coffey, George Nelson. Reconnoissance soil survey of South Texas, book, June 16, 1910; Washington D.C.. (texashistory.unt.edu/ark:/67531/metapth19753/m1/46/: accessed October 22, 2017), University of North Texas Libraries, The Portal to Texas History, texashistory.unt.edu; crediting UNT Libraries Government Documents Department.