Evening star. [volume] (Washington, D.C.) 1854-1972, 1885-07-11

ccWSf AZ Viz hOTtueTQ part OIL the CITY. In the
excavations made in grading the lot bounded by
North Capitol street. NJ street. and New York
avenue,ln the brick clay pitsbetween the arsenal
junction of Virginia and New Hampshire
avenues; on all sides of the elevation upon
which the navai observatory stands; in the Bal
tmore ana Potomac railway cutting east Of the
tunnel; in the brick clay pits on the south side
OF the Potomac near the end OF the Long bridge
tween Washington and Alexandria. except tina,
through the spur just north of Four Mile Run
tn which Inc sands Of the Potomac formation
appear) Except where homogeneous through
out the coarser materials are Invariably below
and the foreign materials become progressively
finer toward the eastern and southern portions
OF the area occupied by the formation until. al
Alexandra, the entire mass, so far as exposed
is an f1n n., homogeneous nos; brick clay.
economic value; the sands are sometimes o
SUFFICIENT purity to be employed by builders
the boulders are generally broken into rubbls
which is used In the construction of foundation
and other walls; the small pebbles are utilized
for road making, and the soil of the formation
is excellent
logically recent origin is attested by its fresh
ness OF aspect and by the little amount of ero
sion that IT mas suffered since its deposition
Moreover, it Is similar in structure, in altitude
and in its relations to subjacent formations to
the brick ciays Of northern Delaware, which
have in turn been satisfactorily correlated with
the brick clays and red gravel of PhiiadeIphia,
These are known to have been laid down about
the close OF the great ice age-the epoch during
dian highlands overspread the northern por.
ton of the United States extending as far
south (in this longitude) as northern Pennsyl.
vania.
The origin OF the materials of the formation
may be aScertained by study of their charac-
,er. me larger bou!ders are mainly identical
lithologicaliy with the Washington gneiss and
are evidently derived therefrom. Certain of
the large boulders are, however, of quartz evil
ing the gneiss The smaller pebbles are mainly
OF quartz and quartzite, and may be separated
into two categories; Those of the first are gen.
angular in form. They, like the quartz bould-
ers, have unquestlonably originated in the
veins occurring within few miles to the west
1nently OF quartzite but subordinately of
quartz, and are well rounded. They appear
to nave been chiefly derived from the gravels of
the Potomac formation, which was broken up
ana redep0sited during the in which the
Columbia formation was laid down. Thus these
quaftZitic pebbles have q varied history, re-
corded in many chapters: the quartz grains of
which they are composed were perhaps quartz
cry in an Archaean granite formed before
sition OF the granite the less obdurate mica and
felspar, suffered decomposition and were
washed into the deeper portions of an early
paieozoicO ocean, while the more refractory quartz
grains were laid down near shore as a sandstone
subsequently altered into quartzite-such
quartzite as forms the Blue Ridge and Sugar
LOat mountains; some OF this quartZite became
broken down under the action of the elements,
was swept into and formed Il shore deposit in
the irlasslc sea; after another change in the re-
lations OF sea and land part of this TriasSic
conglomerate Was broken up and the pebbles
redeposited as portion OF the Potomac forma-
Potomac gravels yielded to the elements and
were In part redepOsited in the Columbia forna-
ton. The finer material of the formation is I'M
sandstone occurring still further to the west
ward, and the sands and clays of the Potomac
formation, together with less quantities of the
detritus born seward from the Appalachian
zone.
The precise mode of origin of the Columbia
gravels and brickclays is suggested by their
geographic position, by their relation to sea
level, and by their composition. 1t will suffice
to say. without discussing the geologic process
involved, that the formation is apparently a
delta deposit laid down in the broad estuary of
the Potomac that existed when the waters of
the ocean rose more than LOO feet above present
So great elevation of the land Tor depression
of the seal in so recent time as this suggestion
implies, appears little short OF incredible to the
man; but it is unequvocably attested by other
INES OF evidence, notably that of the terraces
ahe oa shore lines that characterize the Poto-
mac river from Washington to Its mouth.
THE TOPOGRAPHY OF THE DISTRICT.
The topography of the District of Columbia
and contiguous portions of Maryland and Vir-
ginia is determined SubOrdinately by geologic
structure, but predominently by the action of
destructive geologic agencies, by which the
entire region was first converted into a series
OF extensive plains at various attitudes, and by
which second, these plains We're profoundly
eroded that only narrow mnants OF them now
exist as terraces skirting the Potomac and it's
tributaries. Since nearly every hill and valley
in the District is carved out of eXcavated
into these old plains, 1t will be impossible to
describe all of them verbally; but a few typical
examples may be mentioned.
The lowest important terrace-plain fringing
the Potomac aud vicinity of Wash ingtO1Ai that
upon which the National museum and most of
the southwestern quarter of the city are built
1t is represented on the south side OF the river
broad main frnr SlH-n
lage TO some miles below Alexandria-a plain
which gradually widens from Rosslyn to the
(where it has a width of over two miles,, then
contracts to almost nothing at the mouth of
Four Mile run, widens again to one and a half
miles or more at Alexandria, and again con-
tracts toward Mt. Vernon. It's altitude is about
40 feet above tide. The materials forming the
plain are in general the brick ciays OF the Co
lumbia formation.
The next important terrace is that upon
which the Capitol stands. 1t is exempified on
the Maryland side of the Potomac not only by
Capitol Hill. but by the plain upon which most
Oh the northwestern quarter of the city is To-
Gated. In Virginia 1T is represented by the dis
continuous shelf skirting the bluffs crowned by
Fort Myer and Arlington, and by the wider and
continuous plain extending thence southwest
ward to -Four Mile run, where it has a width OF
fully two miles, but is dissected by Four Mile
run, which has carved out its channel in the
center OF the long eastwardly extending spur
which the plain there forms. Beyond the run
the plain again contracts, and is once more
represented only by a discOntinuous shelf skirt
ing the higher bluffs between Alexandra and
Fairfax seminary. The altitude of the terraces
of this level is about 80 feet above tide. They
are composed of the clays, sands and gravels OF
the Columbia formation, which nowhere ex-
tends much above their level.
The old plains, of which the forty-Goot and
e1ghty-toot terraces are remnants, have been
deeply corraded and largely removed by the
Potomac river and its tributaries: and the
original regularity of contour Of the terraces
has been largely destroyed by artificial excava-
ton.
The next notable terrace Is the shelf skirting
town to Great Falls, occupied in part by the
conduit Through which the city water supply
is conveyed, and the distributing reservoir 1t
Is formed OF Washington gneiss, thinly covered
the secular dsintegraton Of the gneiss, but in
part formed by detritus washed riverward
from the adjacent hills. The old plain of
which this terrace forms part, is, Singuarly
seldom represented in the iinperfecLy con-
sOlidated materials of the Potomac formation:
but one of its remnants appears to form the
plain between lie DrOit Park and Howard Uni
versity, and another forms the spur extending
some hundred yards eastward from the first
angle In the Fort Myer road. this side of the
fort. The altitude of the terrace is about la
feet.
The most conspicuous terrace of the District
Is that forming the highlands just north of the
city. The traveller upon Columbia road, 10th
cannot fail to observe that all OF these roads
occupy portions OF plain of uniform altitude
though the pristine uniformity of the plain has
been destroyed by the corrasion OF three or four
has carved for itself the ravine in which it
flows. The same plain Is represented on the
Virginia side OF the river by the level tract
upon which Fort Myer and Arlington stand
and which extends thence southwark to near
AleAanorla Inc aitltude OF Its remnants is
These Qrn