The citizen. (Honesdale, Pa.) 1908-1914, 1912-09-06
GEOLOG|ST Of
TELLS ABOUT
They Gave Much Trouble,
But Never ImperiIed the
Great Work.

In an article contributed to the En-
gineering Record Donald f. Mac
IA Donald, geologist to the isthmian
canal commission, says that the
great slides which have occurred In the
Culebra cut have never for moment
caused the engineering staff to doubt
the ultimate success Of the great under
taking. He concludes that when the
slopes shall have been reduced to the
proper angle, which will Of course vary
with the strength Of the rock from al
most perpendicular in the case Of
strong lavas to one in five in the case
Of the much sheared clay rocks, the
slide problem will be practically solved
For a time the great masses Of earth
and rock which broke from the steep,
high slopes of Culebra cut and slid into
the excavation We're, to those who re-
lied on the newspaper reports, serious
menace to the successful completion OF
the canal, says Mr.. Macdonald. As a
matter Of fact however, the slides,
vast though they were and are, never
really complicated the engineering
problems of the work. never hindered
the yardage output and never threat
ened the success Of the canal. It is
true that the slides have added much
to the necessary excavation and there
fore to the total cost, but it is a mis-
take to think that they have in the
past or will in the future put in jeop-
ardy the successful completion and op-
Grafton Of the canal.
The slides are due in large part to
the geological conditions. The Oldest
rocks are exposed along the canal be-
tween Obispo and Empire. Faulting,
shearing and weathering have mixed
this already complex mass so that it
is difficult to analyZe 1t. From Empire
southward younger rocks are found in
formation estimated to be $50 feet
thick. These grade upward into light
gray lenses of sandstone from three
inches to three feet thick, separated
from each other by thin beds of shale.
Both series Of beds show remains Of
marine fauna which indicate that they
were originally laid down as sediment
in the bottom Of shallow estuaries Of
the sea, and as the same fossil rela-
tions are found all across the isthmus,
it is evident that the Atlantic and Pa
ciic oceans We're Joined at that time.
Four Types of Slide.
There are four distinct types of
slide, each clearly distinguishable from
the others, yet each aiding and abet
ting the others and all working togeth-
er to pull down material from the high
slopes into Culebra cut to squeeze ma
terial up In the bottom of the cut or to
do both.
These are structural breaks and de-
formations resulting in slides, normal
or gravity slides, fault zone slides and
weathering and surface erosion. The
first class is by far the most important
and troublesome. Fortunately they
scarcely occur outside of the Culebra
district The first manifestation of
these slides is the appearance OF cracks
or assures parallel or somewhat
oblique to the trend of the edge of the
steep slope Of Culebra cut. They ex.
tend from a few yards to a hundred
yards or more back from it and from
each other.
They are usually traceable on the
surface of the ground for several hun
dred yards and gradually widen out
to form perpendicular crevasses sew
eral inches wide. Locally one or more
Of these cracks may appear, and they
may develop into the second stage IN
few weeks or in year or two de-
pending on the surrounding conditions.
The second stage of this type of
slide consists Of canalward fitting of
these blocks, usually accompanied by
a reformation or bulging up of the
rocks in the bottom Of the canal oppo-
site them. There is a downward and n
canalward movement of all the blocks
which reaches a maximum of, say, five
feet to ten feet at the edge of the cut.
There is also a fitting forward toward
the cut on its own axis Of each block.
Takes Year or More to Settle.
These settling, fitting and formative
movements go on for from q few
weeks to year or more before the
third and last stage of these slides is
reached. The last stage consists in the
dropping downward of one or more
blocks, due to the failure and squash
ing out of its base. Then the whole
block disintegrates and soon becomes
a normal gravity earth rock slide, pill
ing up In the bottom Of the cut
This type Of slide is due to the un-
stable geological condition of the rock
formations through which the cut
passes, attributable only to nature and
to the oversteepness and height of the
slopes, blasting and other work attrib-
utable only TO man. Any excavation
removes support from one side of the
column Of earth or rock which forms
the slopes or walls of that excavation.