Evening star., 1952-05-19
Floor Of Granite Gneiss
Supports Capitol City
Busic Rock Extends as Deep
As 700 Feet in Some Spots
By Thomos R. Henry

As 700 Feet in Some Spots
By Thomos R. Henry
Washington has q granite hoor-in
some places buried as deep as TOO feet.
This floor is probably about q half
billion years old. Here and there in the
Northwest there are outcroppings of this
basic rock, but it slopes steadily south
eastward. sinking deeper and deeper
under the accumulation Of sediments
built up through the millenniums.
At some points this bedrock floor is
as much as 600 feet below sea level. It
Is not smooth floor, but has hundreds
Of bulges. ridges. and little hillocks. It
probably would look. stripped bare.
much like q badly warped matched
board floor.
The conflguration Of this floor, worked
out over lifetime by the late Dr. NJ H.
Darton Of the United States Geological
Survey, has just been issued as Survey
professional paper. DR.. Darton obtained
his information from many sources
chiefly boreholes on the sites of the
Washington Monument, Jefferson Me-
mortal. Lincoln Memorial, most of the
new Government buildings. and some
private structures.
Presumably the floor of Washington
can be considered part of the basic floor
Of the world-the rough surface of ut
terly barren rock which existed for
few billion years before the first life
appeared. Actually, DR.. Darton says.
there is no conclusive evidence that this
rock is e-Cambrian-that is. preced-
ing the time of ife-but "the crystalline
rnNkc lo,
any other geological formations in the
region. and their physical properties
contrast sharply with those of the sedi-
mentary formations. Where these crys-
talline rocks have not been affected by
weathering processes they are hard and
either massive or shabby"
Quartz Veins Vary in Width.
The floor can be described broadly as
granite, but more correctly 1t is granite
gneiss. It consists, as does granite, Of
quartz feldspars and mica with small
amounts Of garnet, chlorite. hornblende,
tourmaline and pyrite. This rock, as
described by DR.. Darton, is gray on
fresh exposures, and its generally shabby
character causes it to break more easily
parallel to the banding. Quartz veins
Of varying widths are common. Because
quartz Is resistant to diSintegration by
weathering, large fragments of quartz
may be encountered at shallow depths,
where the inclosing rocks are thoroughly
disintegrated to sandy SOIL"
Upon the floor" various sediment
formations have been piled Oldest Of
these, however, is only Lower Cretace-
ous-at the most about $50 million years
old. These sediments overlie the bed
rock in q wedge-Shaped mass that
thickens rapidly as one proceeds east
ward from Chevy Chase and Tenley.
town.
Among DR.. Darton's observations are
those dealing with such structures as
the Washington monument.
1T is popularly supposed" he said.
shat this is built on bedrock. but this,
unfortunately, is not true. Its founda-
ton. a mass Of concrete about 81 feet
thick and 1861 feet across. lies on sand
and gravel about IS feet above the bed
rock floor. This sand and gravel itself
Is separated from the bedrock by ten to
thirty feet Of blue clay. In 1898 there
was a project to place concrete piers
under the original foundation down to
bed rock but after adding the founda-
? tion and placing under it concrete
collar 13 feet thick, it was thought suffi-
cient stability had been secured. not.
withstanding the yielding nature Of the
underlying deposits. These already had
permitted 5h inches of sinking from
1879 to 1898, but with widened base the
amount Of subsidence from 1900 to 1880
was only about six-tenths of an inch.
The load with enlarged foundation Is
slightly more than $1,000 tons.
Test Boreholes Reach Bedrock.
"The IS test boreholes about the
Washington Monument passed through
gravel, sand. and blue clay. At the top
Is a thick mantle of terrace sand and
gravel. The basal beds lie on low ridge
Of bedrock. The thick body of the blue
clay is regarded by the engineers as
river deposit. Partly petrif1ed wood
In thin layer was reported at AZ feet
below sea level in borehole Cnner
series). about 100 feet due west of the
monument, and thin layer of decayed
below 1t lying on foot of white sand.
Next below is blue clay, here only IL
feet thick. lying on soft bedrock at
-62A feet. old wood was reported at
SO feet on the blue clay in hole z (in-
AACA oOAAVolp 16O0v 4VU ACCt oOQUIlWCot
Of the monument. In hole 'petrif1ed
wood occurs in sand feet above the
bedrock, which is at $50 feet.
''The blue clay, regarded as Pleisto-
Gene or ice age, in age is widespread in
the general region of the monument
and was exposed extensively in the ad-
joining northwest section in excavations
at and Fourteenth Streets, Thirteenth
and Streets, E Street between Thir-
teenth and Fourteenth Streets, Eleventh
Street and Pennsylvania Avenue. and
the blocks between Twenty-first and
Twenty third, c and D Streets. Holes
/ and penetrated mica rock for 20
feet. Holes /, 3, 4 and penetrated soft
rock grading down into hard rock for
20 to Is feet. Hole reached blue clay
at go feet. under about So feet Of
sand and gravel, and penetrated 1t 8
feet to within about is feet of the bed
rock. Hole reached blue clay about
-27 feet. Hole Ga was In blue clay from
-23 to -58 feet. with small amount
of gravel and mica at base"