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286 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Sierra Pelona. The volcanic rocks are similar to
present as clasts in the conglomerates of the Mint
flows in the Vasquez, east and northeast of the area. Canyon formation are especially indicative of an
The lithologic nature of the Mint Canyon formation easterly derivation of the sediments, but the presence
in the mapped area is not characteristic of the entire in some of the conglomerates of schist and sandstone
formation in adjacent areas. The “light-gray to clasts similar to the Pelona schist and sandstones of
nearly white gravel interbedded with greenish clay or Eocene age in the mountainous region north of the
fine sand” (Kew, 1924, p. 52) in the upper part of the Santa Clara River valley indicates a northerly source
formation does not occur in significant amounts in for part of the formation.
the mapped area. A thickness of about 1,500 feet During deposition of the Mint Canyon formation,
of the Mint Canyon formation is exposed in the mapped the region was probably a large alluvium-covered
area. The base is not exposed. valley or plain with scattered lakes, tree-bordered
streams, and grass- and brush-covered alluvial-fan
ENVIRONMENT OF DEPOSITION
surfaces leading up to the adjacent mountainous
The Mint Canyon formation was deposited under 8 teaS.
subaerial conditions. The coarse, unsorted, and len
MODELO FORMATION
ticular sand and conglomerate beds appear to have
been large alluvial fan deposits. Fresh-water lakes The Modelo formation was named by Eldridge and
were present in the area concurrently with the develop Arnold (1907, p. 17–19), who described as typical its
ment of the alluvial fans. Thick sections of siltstone exposures in Hopper Canyon and at the head of
* †
with interstratified mudstone, tuff, sandstone, and Modelo Canyon in Ventura County. As originally
conglomerate are lake deposits. The presence of defined, the formation consisted at the type area of
fresh-water mollusks and of a turtle possibly related from 1,700 to 6,000 feet of strata, divided into four
to Clemmys (Maxson, 1930, p. 82, 87) is evidence of members: a lower sandstone member, 200 to 2,000
a lacustrine environment during deposition of part of feet thick; a lower shale member, 400 to 1,600 feet
the formation. Maxson regarded the abundant re thick; an upper sandstone member, about 900 feet
mains of hypsodont horses, antelopes, camels, and thick; and an upper shale member, estimated to be
rabbits as an indication that the vegetation of the between 200 and 1,500 feet thick. Eldridge and Arnold
region must have been at least as abundant as that described the formation as overlying a shale sequence
supported by a semiarid region. The grazing types of correlated with the Vaqueros formation and underlying
mammals occupied grass-covered plains, while Para rocks of the Fernando group. Kew (1924, p. 55–69)
hippus, peccaries, and possibly the oreodonts and redefined the Modelo formation to include the under
mastodons arso found in the formation frequented lying beds correlated by Eldridge and Arnold (1907)
wooded areas along streams and lakes. Axelrod (1940, with the Vaqueros. As thus redefined, the Modelo
p. 577–585) made a study of fossil plants from tuff beds formation at the type area consisted of five members:
of the Mint Canyon formation in the vicinity of a lower shale member, a lower sandstone member
Bouquet and Sand Canyons. He found elements in (the basal unit as defined by Eldridge and Arnold),
the flora indicative of at least four distinct habitats: a middle shale member, an upper sandstone member,
rush- or reed-like plants suggestive of shallow lakes, and an upper shale member. The aggregate thickness
desert scrub from the drier slopes of the lower basin, of this sequence was given by Kew as about 9,000
an oak assemblage from the savanna area surrounding feet.
the general basin and probably also from the borders Hudson and Craig (1929) redefined and restricted
of streams, and a woodland community found at the Modelo formation at the type area. They cor
higher altitudes and on cooler slopes. Comparison related the lower shale, lower sandstone, and middle
with similar modern floras indicates that the region had shale members of Kew's Modelo formation with the
an annual rainfall of from 15 to 20 inches, that pre Topanga formation, largely on paleontological evi
cipitation was distributed as summer thundershowers dence. They also excluded from their restricted
and winter rains, and that temperatures were similar Modelo the uppermost beds of Kew's Modelo formation
to those now prevailing in the region, except that the at the type area.
winters were slightly warmer. The members of the Modelo formation at the type
The source of a large part of the Mint Canyon sedi area are not recognizable in the Santa Susana Moun
mentary beds was probably to the east, where rocks tains. In the area included on the geologic map
similar to the types represented as clasts in conglom (pl. 44), the Modelo formation consists mainly of rocks
erates of the Mint Canyon formation occur in the San that Kew (1924) mapped as a shale member of the
Gabriel Mountains. Anorthosite and related rocks Modelo. Strata mapped by him as an overlying sand