none, Historical Area Geology Report, Studies & ReportsJOHN A. LAMBIE
COUNTY ENGINEER
COUNTY *OF LOS ANGELES
DEPARTMENT OF COUNTY ENGINEER
108 WEST SECOND STREET
LOS ANGELES, CALIFORNIA 90012
629.4747
CITY OF ROLLING HILLS'
August 7, .1968
HARVEY T. HRANDT •
CHIEF DEPUTY
LAWRENCE D. MOORE
ASST. CHIEF DEPUTY
WILLIAM A. JENSEN
ASST. CHIEF DEPUTY '
ROBERT K. WILLIAMS
ASST. CHIEF DEPUTY
The City Council
City of Rolling Hills
Rolling Hills, California
Gentlemen:
Enclosed, as requested, is an evaluation "in depth"
of Dr. Natland's geologic report regarding the "Roach
area."
Very truly yours,
John A. Lambie
CITY ENGINEER
of
JAL: AGK: fe 7
Enc.
JOHN .A. LAMBIE
COUNTY ENGINEER
COUNTY OF LOS ANGELES
DEPARTMENT OF COUNTY ENGINEER
106 WEST SECOND STREET
LOS ANGELES, CALIFORNIA 90012
629.4747
CITY OF ROLLING HILLS
August 7, 1968
REVIEW OF DR. NATLAND'S GEOLOGIC
• REPORT OF THE ROACH AREA
CITY OF ROLLING HILLS, CALIFORNIA
HARVEY T. BRANDT
CHIEF DEPUTY
LAWRENCE D. MOORE
ASST. CHIEF DEPUTY
WILLIAM A. JENSEN
ASST. CHIEF DEPUTY
ROBERT K. WILLIAMS
ASST. CHIEF DEPUTY
As requested, we have reviewed Dr. Natland's report,: and while
recognizing his many distinguished contributions to geology
throughout his long career, we are, nevertheless, required to
accept only reports from certified engineering geologists
when such reports are required by Ordinance. This review is
an evaluation only and cannot be considered as an official
report in connection with approvals or disapprovals of sub-
divisions located on the property in question.
In the present case the property was mapped by the State of
California, Division of Mines and Geology, for the County and
designated as an ancient landslide area. Detailed .investiga-
tions by certified engineering geologists have subsequently
more closely defined the boundary of the northern slide area,
but have not changed its designation.'
That which follows are discussions indicating why Dr. Natland's
report is not considered conclusive.
CORRELATION OF MARINE TERRACES
According to United States Geological Survey Professional, Paper
No. 207, terrace correlations asshown on their map (Plate 2)
are far from satisfactory, and other correlations may be equally
or more plausible,. In the absence of accurate rear terrace
altitudes (at foot of sea cliff) on numerous platforms, correla-
tions are dubious. Furthermore, correlation of terrace remnants
is doubtful. Additional reasons why terrace correlation in the
Palos Verdes Hills is very difficult are as follows:
1) Tilting of terraces due to contemporaneous and/or post
terrace arching is especially evident in the northeast
part of the Palos Verdes Hills and is related to continu-
ous mobility along the Palos Verdes fault.
2) Designation of terraces was not based entirely on platform
altitudes. The apparent merging of terraces of different
ages. clearly shows this. Mapping terraces is very difficult
due to masking by thick nonmarine deposits and residual soils.
3) Terrace discontinuity 'oi slopes due to submarine scouring
and mass wasting, e. g., landslides, soil creep, subareal
erosion.
4) Poor terrace development on leeward slopes.
5) Varying thickness of'nonmarine 'cover (0 to 100'+) as
well as multiple nonmarine cover.
Intermediate terraces such. as 7a'and 5a were not 'corre-
lated around the hills; therefore, one or both could be
present elsewhere besides at the type locale.
7) "Many marine terrace platforms may have been totally or.
partly destroyed by subsequent terrace development.
$) Range of apparent altitude at the rear of successive
terraces is from 75 to 200 feet. '
:Et is, therefore, concluded that Dr. Natland's seventh terrace,'.
correlation acoss the 'Roach area is highly idealized especially,
when you consider (a) that the terrace platform on the Hayward.
pit is'tilted .15°±-to the north (See photo page 23); (b) that the
terrace platform below the terrace deposit along' Roadrmner, Road
was never found and assumed by Dr. Natland to be at,elevation 650;
(c).that 10 feet ofnonmarine cover and no marine terrace platform.
was found in the $0 foot 'deep Roach boring and just a, few feet
away, at the same ground elevation,, the. Hayward pit encountered
a tilted marineterracedeposit 5 feet' below ground surface; and
(d) that only one marine terrace locale has been uncovered in
all the subsurface explorations conducted in the Roach area.
ABSENCE OF LANDSLIDE INDICATORS IN ROACH AREA
It has never been the opinion of the Engineering Geology Section
or the' State Pivision of Mines and Geology that the Roach:. area
is an active landslide. Dr. Natland's observations regarding the.
lack ofoutstanding landslide features and distress to structures
(typical of the active portion of the Portuguese Bend slide) are
justified. However, damage to structures has not occurred within
the ancient landslides which are considered inactive.
The State Division of Mines' geologic map is the result of a
regional study by Dr. R. H. Jahns and'G. Cleveland of the Palos
Verdes Peninsula to delineate geologic features of possible
engineering significance. The Roach area (as were the marine
terraces) was mapped as an ancient inactive landslide mainly on
the basis of physiographic and surficial geologic evidence.
Because the geologic mapping was not basedon subsurface explora-
tion, subsequent detailed geologic and soil engineering studies
by private consultants have warranted modification, of the map.
Recent comprehensive studies by private geologists have enabled
the Engineering Geology Section to modify the northern boundary
of the Saddleback slide. The southern portion, a highlycomplex
area geologically, has notbeen re-classified because private
consultant's investigations have encountered additional ancient
landslide indicators, and as a result leave many questions
unanswered. Following are geologic factors suggestive (some..
highly suggestive) of ancient landsliding; either pre- or post-
seventh marine terrace deposition:
1. Borings on Margrave property encountered an ancient debris
or talus -filled trench or canyon suggestive of a slide
pull -away.
2. Soil zone of variable 'thickness (li to 26 feet) suggestive
of.soil filled sag ponds which are typical of ancient
landslides.
3. Landslide -prone schist breccia'zone (inclined:10°-25°
northerly) presentat head of bowl and approximately
65 feet below the Griffith ,'andMcCall properties as
determined by Lockwood.
4• Adverse orientation of schist (basement rock) foliation.
5. Diverse attitudes of Altamira'shale member in borings,`
trenches, -and on outcrops:: along Saddleback,. Road.
6. Shallow ground water present on Saddlebackarea:,
a. Beeks--42 feet below ground. surface.. .
b. Roach--60 feet below ground' surface.
c. 'Griffith--51feet below 'ground' surface.
7. Presence of slickensides, clay-filledfractures and ben-.
tonite as found in boringsand trenches.
$,. Absence of correlatable geologic. marker beds across the
Saddleback area, e.g., Miraleste tuff, siliceous and
cherty.shales.
9. Poorly defined and highly questionable continuity or cor
.relation of marine terrace deposits' 'and platforms. Greatly.
tilted marine terrace platform as indicated on photo of
;Hayward trench. '
10. Anomalous topography, .100-foot'high bowl shaped "scarp" per.
USGS topographic map and 1928 aerial photos.
Relatively flat profile below "scarp" typical of all
landslides.
12. Subdued hommocky topography per 1928 aerial photos.
13. Anomalous drainage courses normal to regional ground slope.
14. Well defined typical landslide topographic features are
not necessarily a primary criterion in landslide identi-
fication as evidenced by small ancient slide found on BeeksY-
property. This slide had no topographic expression except
for, the offset drainage course at toe.
15...The .7th marine terrace platform has not been encountered on
Beeks' and: other properties in the Roach area. If the
7th terrace was very broad, as suggested by Dr. Natland,
better preservation seems probable.
GEOLOGIC STABILITY OF ROACH AREA
A significant finding made by Maurseth-Howe-Lockwood and Asso-
ciates is that a stability analysis along a probable slide
plane (on basis of all available information) indicates that
the Roach area is not a geologic hazard under present day con-
ditions of subsurface moisture and ground configuration. Since
the exact driving and resisting forces are notknown, and also
sincethe analysis does not assume saturated soil conditions,
the Engineering Geology Section approved Tract 22083.(Beeks)•on
the basisthat additional water introduced by conventional
seepage pits not be permitted within the landslide mass.
FEASIBILITY OF PRIVATE SEEPAGE PITS
According to. Dr. Natland's preliminary data, certain types of
vegetation are capable of removing large amounts of. water 'from
the ground. It must be stressed that a more detailed hydrologic.
study that accounts for all imported and exported water is needed
prior to considering private seepage disposal systems. According
to Palos Verdes. Water Company sources, the average household in
Rolling Hills consumes on the average; of 200 gallons per person
perday., or25' percent more than assumed by Dr. Natland. A hydro-
logic study shouldutilizethe most realistic estimates available.
John La'•ie
CITY ENGINEER
JAL:AGK:RMR 7
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