CULTURAL GEOGRAPHY OF THE MODERN TARASCAN AREA—WEST 33 
historical examples of vulcanism within or ad- 
jacent to the area. However, vulcanism associ- 
ated with human ancient settlement in the area, 
is unquestionably evidenced by the occurrence of 
charred maize ears embedded in a basaltic lava 
flow 18 km. northwest of Morelia.* 
Aside from land forms resulting directly from 
vulcanism, others caused by depositional processes 
occur within the Sierra. Between the old com- 
posite volcanoes and young cinder cones flattish 
surfaces have been formed first by ash and cinder 
fall and later by aeolian and alluvial deposition 
from surrounding slopes. Such surfaces, which 
compose only a small percentage of total land 
area, are the agriculturally important sections of 
the Sierra. Some form saddles or cols between 
adjacent hills; the larger plains form basins of 
interior drainage. The largest basins are Llano 
Grande, southeast of Charapan; the Plan de 
Nurio; that of Paracho, Aranza, and Cheran; the 
plain of Nahuatzen; of Sevina; of Paracho; ete. 
(map 2; pl. 1). Minor forms include dissected 
aprons of alluvium and semiconsolidated ash at 
the base of volcanoes and steep-sided arroyos 
eroded into alluvium in the upper parts of the 
basin plains. 
One of the most characteristic physical features 
of the Sierra is the paucity of perennial streams 
and lakes. Although this is an area of moderate 
summer precipitation, the porous volcanic surface 
quickly absorbs most of the available moisture. 
A few permanent, spring-fed streams occur on 
the northern and western flanks of Cerro de 
Patamban and on the western and southern sides 
of Cerro de Tancitaro. Small springs occur on 
the flanks of the old composite volcanoes, whose 
porous rocks form a reservoir of rain water. Such 
springs afford the sole water supply for many 
Sierra villages. Only on the edges of the Sierra 
plateau, at the contact of porous and impervious 
rock layers, do large springs occur. 
The Lake area.—At the eastern edge of the 
Sierra exists a depression filled by Lake P&tzcuaro. 
Sierran geomorphology (young and old volcanic 
forms) almost surrounds the lake, but predomi- 
nates on its northern, western, and southern shores. 
Twelve kilometers south of Pétzcuaro small Lake 
Zirahuén, within the eastern prong of the Sierra, 
appears to have been formed by a lava-blocked 
4 An excellent specimen of charred maize so embedded is in the Museo 
Regional Michoacano, Morelia, 
stream.’ Both lakes present similar shore fea- 
tures—moderately steep banks alternating with 
wide, flattish delta fans. 
In other respects the two lakes are distinct. 
Patzcuaro is shallow (deepest point in the southern 
arm, 20 feet; in the northern part, 50 feet, in 
1941); Zirahuén is relatively deep (148 feet, maxi- 
mum sounding). Moreover, while the level of 
Zirahuén (6,953 feet in 1942) is relatively stable, 
that of Patzcuaro (6,671 feet in 1942) has fluctu- 
ated frequently in historical times. Since 1939 
the lake level has descended nearly 4 feet, exposing 
large areas of mud flats along the shore and re- 
vealing one new island (Pastora) near the south- 
west corner of the lake (pl. 1). In some places 
the descending waters have uncovered old tree 
stumps, indicating that formerly the lake level 
was even lower than at present. On the other 
hand, the western portion of the large lacustrine 
plain of Chapultepec in historical times formed 
an eastward extension of the lake’s southeast arm, 
denoting a former level higher than the present 
one.® The cause of flucuation in level is not clear. 
De Buen (1944), judging from meteorological data 
(1939-43), has suggested a correlation between 
rainfall and evaporation variation on the one 
hand and lake surface fluctuation on the other. 
The excessive evaporation and low rainfall during 
those years likely affected lake volume, but normal 
precipitation and evaporation in following years 
have not halted the gradual decrease in lake level. 
Moreover, if fluctuation is caused by meteorolog- 
ical elements, one would expect neighboring lakes 
to be similarly affected. Periodic clearing and 
clogging of sublacustrine crevices, often associated 
with lakes of volcanic origin, is another popular 
hypothesis applied to Lake Patzcuaro. 
The lake contains 10 islets on which live some 
of the most conservative of the Tarascan people— 
the fishermen. The four islands off the Taafu-k*éri 
Peninsula (Pacanda, Yunuén, Tecuena, and 
Janitzio) are old volcanic hilltops with steep banks 
and, with the exception of flat-topped Pacanda, 
with little land suitable for cultivation. In the 
5 De Buen (1944) assigns the origin of all present and former lakes in the 
western Portion of the Mesa Central to disruption of normal drainage by 
vuleanism. He also suggests that Lakes Patzcuaro, Zirahuén, Cuitzeo, and 
Yuriria were formed from portions of Rio Lerma drainage, since the fish 
Chirostoma, abundant in Lake Chapala and the Lerma, is found also in these 
lakes, the number of species decreasing from Chapala (10) to Zirahuén (2). 
6 The 16th-century map of Lake PAtzcuaro which accompanies the Relacién 
de Michoacan (1903) clearly shows the former extent of the southeastern arm. 
At the time of the Conquest this section included the island of Aputato, now 
a hill some 300 m, from the water's edge. 
