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GEOLOGY OF SOUTHEASTERN VENTURA BASIN 279

Mountains. The younger terrace surfaces are at alti Chiquito Canyon follows closely the trend of the Holser
tudes as low as 900 feet along the river at the western fault for about 2 miles before making a bend, crossing
border of the area and as high as 1,900 feet on the hills the fault, and joining the Santa Clara River. Simi
near the eastern border. larly, the course of the upper part of San Martinez
Many small streams of the Santa Clara River drain Grande Canyon may have been originally determined
age have their sources near the crest of the Santa Susana by the position of the nearby Del Valle fault. The
Mountains. Some of these streams have been be canyon parallels this fault for nearly 2 miles before
headed by other streams which originate on the oppo crossing it to join the Santa Clara River.
site side of the mountains and are tributary to the San
RIVER, TERRACES AND OLD EROSION SURFACES
Fernando Valley drainage.
Several river-terrace levels and old erosion surfaces
Landslides are conspicuous. They are found in the
Santa Susana Mountains on steep slopes underlain by are exposed along the Santa Clara River and its trib
shale and siltstone and in the northwestern part of the utaries. The hills north of the river between Bouquet
mapped area on high steep hills. Canyon and the eastern border of the mapped area
are marked by remnants of seven distinct river terraces.
STRUCTURAL AND LITEIOLOGIC CONTROL OF
Figure 50 shows diagrammatically the spatial distri
IDRAINAGE
bution of these terrace remnants and their correla
Because the Santa Clara River valley is developed tion. The terrace surfaces are formed on both river
more or less along the axial trend of the Ventura basin, terrace deposits and rock of the Saugus formation.
the rocks exposed near the center of the valley are gen The geologic map (pl. 44) shows the extent of river
erally younger and less resistant to erosion than are terrace deposits but not necessarily the extent of sur
those nearer the basin margins. The structural de faces formed on these deposits. Inasmuch as the
pression of the basin, together with its attendant faults, river-terrace remnants depicted on figure 50 have only
folds, and the accumulation of easily eroded younger a thin veneer of terrace deposits, the limits of the de
rocks near the center, is largely responsible for the po posits as shown on the map approximate closely the
sition of the valley. extent of the terrace surfaces as well. The lowest
The pattern of the tributaries to the Santa Clara terrace surface is about 40 feet and the highest more
River in several parts of the mapped area is controlled than 400 feet above the present river bed. These ter
by lithology and structure. Sedimentary rocks ex race surfaces are generally nearly planar with gentle
posed on the northeastern flank of the Santa Susana slopes toward the river, but some steepen against the
Mountains have a strike nearly parallel to the trend of hills. The highest terrace surface extends back into
the mountains themselves. The thicker conglomerate the highlands up gently sloping valleys unrelated to
and sandstone units are represented by strike ridges the present drainage. Surfaces reconstructed from
and the thicker siltstone units have been eroded to form
the remnants of various terrace levels slope in approx
the intervening valleys. Dip and antidip slopes are imately the same direction as the present river valley.
commonly almost equally steep so that the longitudi A comparison of the gradient of the river that formed
nal or strike valleys are narrow canyons. Some major the terraces with the gradient of the present river is
drainage lines, such as Pico Canyon, begin in amphi difficult to make because of the limited extent and
theaters high on the north side of the main ridge of the distribution and the possible deformation of these ter
Santa Susana Mountains, follow the strike for distances
race remnants; however, the gradients of the streams
as much as 1 mile, and then alternately cut across the that formed the two highest terrace levels appear to
strike or follow it until they reach the lowland region have been flatter in this area than that of the present
west of Newhall. The canyons are narrowest where river (fig. 50).
they cut through strike ridges. Towsley Canyon, for Remnants of an old erosion surface are found above
example, is deepest and narrowest where it crosses a the river terraces north of the Santa Clara River. This
unit of hard sandstone and conglomerate with a se surface is on and near the tops of the highest hills and
quence of less resistant, finer grained rocks in the middle. is as much as 520 feet above the present river bottom.
The gorge crosses the resistant rocks, makes a right South of the Santa Clara River, directly opposite the
angle bend to follow the softer intermediate beds along area of extensive terraces, only one terrace remnant
the strike for 400 feet, and then turns again at right is preserved. No correlation between it and those
angles to cross the remaining hard beds. north of the river is apparent. Between Dry Canyon,
The positions of several valleys seem to be deter just west of Bouquet Canyon, and Castaic Valley, no
mined in part by the locations of major faults. In the terraces are present along the north side of the central,
northwestern part of the mapped area, San Martinez alluvium-filled valley of the Santa Clara River.