Page 22 - wintererdurham1962

290 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

destroyed the foraminiferal tests. Where the rocks Some of the mappable sandstone and conglomerate
are well bedded or where foraminiferal concretions are units can be traced laterally for only a few hundred
present, interstratal solution probably destroyed all yards. Others, like the resistant unit that is so well
calcareous shells except those protected within exposed north of the axis of the Pico anticline where
concretions. Towsley Canyon narrows to a gorge, can be traced for
At a few places in the Towsley formation thin units several miles. Even this persistent unit is variable in
of yellowish-gray shale with paper-thin laminations both lithology and thickness from place to place. At
crop out. In these laminae fish bones and scales are the narrows in Towsley Canyon it is about 225 feet
abundant, and well-preserved mud pectens (Delecto thick and consists largely of conglomerate. Clasts
pecten sp.) are common. near the base of the unit are as large as 18 inches in
Sandstone and conglomerate–In many areas, the diameter. One mile northwest of Towsley Canyon,
sandstone and conglomerate units are firmly cemented near Little Moore Canyon, the unit is only a few feet
so that they produce bold topographic forms. Many thick and consists of sandstone. At Little Moore
streams have waterfalls in their courses where they Canyon the unit thickens and coarsens abruptly again,
cross these resistant units. The dense cover of brush then gradually thins toward Pico Canyon. South of
that grows on most sandy slopes is nearly impenetrable the axis of the Pico anticline in Towsley Canyon the
in some areas, especially along the Santa Susana same unit consists of coarse-grained sandstone.
Mountains in the upper reaches of Towsley and Pico The clasts of pebble or larger size in the conglomerate
Canyons. Many of the sandstone and conglomerate represent a great variety of rock types. At most
units contain large concretions such as those shown in places gneiss and leucocratic plutonic rocks constitute
figure 51. Bedding planes pass through the concre about 80 percent of the clasts. Dark volcanic rocks,
tions without interruption. Individual beds within chiefly andesite but representing a wide range in com
these mappable units range from thin laminae to beds position, generally make up about 15 percent, and a
at least 5 feet thick. At many exposures single beds host of other rock types, including quartzite, vein
several feet thick pinch out completely in a few tens of quartz, chert, aplite, gabbro, norite, and anorthosite,
feet. The beds of conglomerate tend to be the most constitute the remainder.
variable in thickness.

FIGURE 51.-Concretionary sandstone unit in the Towsley formation on the crest of the Santa Susana Mountains.