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17.

The large exposure of "Modelo" formation that des-

cribes an arc in the southeastern corner of the area forms

steep bluffs as much as 200 feet high (Figure 3J. ~he beds
reach thicknesses of 10 and 15 feet \Figure 4J. buff and tan

colors with rusty streaks predominate. Sandstone and siltsnne

with shaly layers make up most of the strata. Limy concretions,

elliptical in shape, are common at certain horizons, generally

in siltstone beds. Clayey diatomaceous shale beds also are
present.

The ''Modelo" formation was deposited under brackish-

water and marine conditions, when an arm of the sea trans-
gressed over this region in Upper Miocene time.

Pico Formation:

Four exposures of the Lower Pliocene Pico formation
have been recorded on the accompanying map. ln the large one,

toward the east side of the area, the Pico formation uncon-

formably overlies the "Modelo" formation in a broad synclinal
basin with very gently sloping sides. The northwestern edge

of this exposure forms high bluffs with a badland topography,

and, as shown in figure 2, overlies the wedges of "Modelo"

shale, siltstone and conglomerate with an angular discordance of
only a few degrees. The southeastern part of this exposure

is not as thick stratigraphically as the above portion. It

may be seen in figure 4 overlying the ''Modelo" cliff in this

region.
The next exposure westward lies between two faults and

rests directly upon Mint Canyon beds. It is a rather thin

capping on a ridge. The third exposure, near the fire depart-

ment road on section B-B', forms northward facing cliffs. It

cannot be traced for any great distance.