CULTURAL AND HISTORICAL GEOGRAPHY OF SOUTHWEST GUATEMALA—McBRYDE Sil 
APPENDIX 1 
SUMMARY NOTES ON THE 
PHYSICAL ENVIRONMENT 
GEOLOGIC FOUNDATION OF THE LANDSCAPE 
(Map 5) 
In their broad features, the physical landscapes of South- 
west Guatemala reflect to a high degree the underlying 
geologic make-up of the region. The Pacific Coastal Plain 
(costa), with its frayed, recently emerged lagoon shore, 
consists for the most part of alluvial material (V. Al., map 
5) derived from the parallel volcanic Highlands which loom 
to the north and rise abruptly from the Lowlands in the 
form of a nearly straight inner range, trending northwest- 
southeast. This southwestern margin of the elevated interior 
is fringed with strikingly steep, young eruptive cones of 
andesite, ash layers alternating with lava flows (V., map 5). 
There is a disparity between the highest points of elevation 
and the Continental Divide. The latter is formed by an 
older volcanic range (Los Altos, V’., map 5) with its axis 
parallel to and inland from the recent volcanic chain; many 
of the cone summits rise higher than the broadly undulant 
crest of the Altos, the average elevation of which is greater, 
however, than the file of recent volcanoes. The trough be- 
tween the two ranges, which coincide only at the great cone 
of Tajumulco, highest point in Central America, is filled 
with immensely deep deposits of pumiceous ash and breccia. 
Great irregularities characterize this zone, with basins which 
may contain lakes such as Atitlan (map 20) and Amatitlan, 
or wide expanses of nearly level plains composed of fine 
fragmental ejecta like that of Quezaltenango (pl. 38, a); 
between such basins rise great ridges of resistant volcanic 
material such as lava flows from recent cones. The interior 
trough country of Huehuetenango-Sacapulas-Salama (pl. 
42, e) is made up predominantly of ancient crystalline rocks 
(Cr., map 5), especially granites and gneisses, which appar- 
ently form a basal horizon underlying much of the more 
recent, superimposed volcanics to the south. In this older 
region of Antillean structure, the fold-axis is east-west, the 
strike of which is reflected in the master streams, as the 
Cuilco and Negro. These rivers follow close upon the line 
of contact between the crystallines on the south, and the 
calcareous sedimentaries of the tremendous, sharply elevated 
horstmassif of the Altos Cuchumatanes, directly to the north 
(Ls., map 5). 
The Highlands are everywhere deeply dissected, though 
the barrancas, or gorges, that attain immense depths, are 
most steep-sided and abrupt in the region of fragmental 
eruptives, where loesslike ash retains high, vertical faces. 
Short, rapid, nearly parallel rivers are very numerous along 
the rainy Pacific versant (map 1). The greatest canyon 
profundity is attained where antecedent streams flow be- 
tween volcanic cones. Erosion is at a maximum where steep 
initial slopes (formed by eruption or faulting) lend high 
velocities to the torrential wash of heavy rainy-season down- 
pours, and unconsolidated material has been exposed by 
agricultural clearings. Such combinations of factors are not 
at all uncommon in the more populous sections of the High- 
lands (pls. 26, a; 45, c). 
CLIMATE AND WEATHER 
(Map 6) 
Climates in native terms, as generally classified in moun- 
tainous Latin American countries within the Tropics, are 
three: Tierra caliente (hot) roughly 0-1,000 m. (3,280 ft.), 
tierra templada (temperate) 1,000-2,000 m. (6,562 ft.), and 
tierra fria (cold) over 2,000 m., the limits being somewhat 
vague and inconclusive as regards natural vegetation and 
agriculture. There are two seasons recognized locally: 
“Verano” (dry) and “invierno” (rainy), in which the words 
“summer” and “winter” are actually applied in reverse to 
seasons of low sun and high sun, respectively. In terms of 
the Kdppen system, there are three major climatic belts, a 
broad Lowland tropical savanna (Awgi) (see map 6 for ex- 
planation of symbols), extending upward into the Lake 
Atitlan Basin through the low gap of San Lucas; a nar- 
rower tropical monsoon (Amw’”i) along the piedmont and 
orographic front exposed to marine winds; and a meso- 
thermal winter-dry (Cwg), extending throughout the High- 
lands, above an average elevation of about 1,450 m. Sea- 
sonal ranges of temperature are everywhere slight, though 
diurnal extremes are generally high, especially during the 
clearer dry season. This almost rainless verano lasts from 
early November through April in most of the region except 
in the monsoon belt, where it is reduced to a December- 
March period of low rainfall, not, however, so dry as in 
other climates above and below the wet zone. The band 
of heaviest precipitation here along the Pacific slope is be- 
tween about 700 and 1,400 m. (2,297 ft. and 4,593 ft.) ele- 
vations, where humid marine air is cooled below conden- 
sation temperatures by orographic ascent. There is a distinct 
double maximum (June-September), with most of the rain 
coming in heavy afternoon and evening showers. The pre- 
vailing day-time wind of the Highlands is southerly 
(marine), beginning about the middle of the morning, and 
accompanied by heavy cloudiness above about 2,000 m., even 
during the dry season. Only on days of high pressures and 
strong northerly winds, commonest from December through 
March, are skies exceedingly clear all through the day. The 
pronounced diurnal wind-shift ordinarily brings such north 
winds (‘“land-breeze,” “mountain-breeze,” and trade-wind, 
combined) at night, through most of the year, except during 
wet season “temporales,’ or prolonged, almost uninterrupted 
spells of rainy weather. Normally, after sunset the massive 
cumulus banks associated with afternoon convection are 
rapidly dissipated or swept southward, leaving the sky clear. 
(For a more complete discussion of climate and weather, see 
McBryde, 1942 a and 1942 b.) 
