Evening star. [volume] (Washington, D.C.) 1854-1972, 1885-06-20

more than 800 feet at Washington.
THE TERRANEs OF THE POTOMAC VALLEY.
The rock masses traversed by the valley of
the Potomac are shown in their natural order
by the accompanying scheme.
Quatenary. t?).;Columbia.
csn010'e. Se0se"e........ S G,9sins:
Mesozoic. Trisssie' =; f Potomac
( Carboniferous.
r''s''''s. 5 sSS;$I$s:
Azoic C) Archaen ~
The Columbian gravels and brick clay forma-
ton. the normal phase of which is we'll ex-
hibited in the great boundary sewer near the
deaf and dumb asylum. in the grading of the
North Capitol street and LIST street, and exca,
cavations on Virginia avenue, near the Observa-
tory, is described by Mr.. McGee as essentially
mixture of clay fine sand and boulders more
or less distinctly stratified; but locally it may
gravel and bouIders imbedded in a matrix of
clay, or q tripaTtite mass. consisting Of brick
clay superiority, stra.ified sand medially, and a
eXemplified at the brick yards between the
arsenal and navy yard in the southeastern
south side of the Potomac s near the end of the
Long bridge. The gravelly phase is well ex-
hibited in the street, cellar and other excava-
tably in the cellar of the unfinished casino
building on Connecticut avenue and Boundary
phass wss VEIL in The
for the Moses building, corner of and 18th
streets, in the few brick yards in which the cut
cuns ea t1 of Nayv 1ed and aee
far as known, the deposit is unfossilifcrous.
The formation is confined to attitudes not ex-
over the portion Of Washington, limited on the
north by Boundary street and over the broad
low plains south of the Potomac in Alexandria
county. Va. 1t also occurs on the east side of
the East branch and of the main river, below
the bend at the arsenal. 1t is referred to the
quaternary, though with some doubt,by reason
of its lithologic and altitudinaI identity with
the brick clays and gravels of northern Della
ware, which are in turn correlatable with tue
Lewis, which are known to represent the later
stages OF the quatenary. The neocene forma-
of the river beyond the Matthias point and con
sist mainly OF little consolidated clays and
sands dipping slightly to the eastward. The
Eocene formation, mainly composed OF sands
and clays, with occasional calcareous layers, is
enthusiastically developed upon the Patuxent,
on the piscataway, in the submits Of the bluffs
overlooking the Potomac river at Fort Wash
ington, and in the river bluffs east of the
mouth of the Acquia creek.
h, Inc Potomac formation, exemplIfied in the
Dlu11S bounding the basin in which Washing
ton Is situated, and along both shores of the
Potomac as far south as the mouth of Acquia
vario,,sly combined: is without coniimuoms,
qualification and is unfossiliferous.
~, "A considerable portion of the plain section of
the Piedmont division of the Potomac valley is
noored by sandstones and states and conglom-
crates which have generally been referred to
the Irasslc. The paleozoic rocks, of which the
Appalachian system is composed, consist of a
succession OF sandstOneS, limestOnes and Shales
several thousand feet in thickness. The azoc
group comprises the gneiss, mica schists and
related crystallines found in the plateau Sec
ton Of the Piedmont division Of the valley
TERRACES OF THE TIDE WATER DIVISION.
Mr.. McGee described a few typical terraces of
each of the divisions Of the Potomac valley be-
ginning with the terraces of the tidewater divi-
SION. Conspicuous among the terraces firing
Ing the Potomac in the vicinity of Washing
ton," he said, sis that upon which the Capitol
was located. 1t is represented on the north
side OF the river not only by Capitol Hill, but
also by the plateau upon which the northwest
portion OF the city is built. On the south side
skirting the bluffs upon which is located the
United states cemetery ut Arlington, and
stretching THENCE southeastward TO Four-Mile
tun. The Old plain OF which it is the remnant
has been largely removed by the Potomac river
ana ITS tributaries; and the original regularity
stroyed by artificial excavation. The Terraces
are notable, mainly in that they are terraces of
construc.ion-wherever preserved the plain Is
formed OF the ci3yS, sands and gravelS Of the
Columbia formation, which nowhere extends
much above its level The materials of this
formation are evidently of sub-aqueous origin,
and their coarseness and heterogeneity would
Indicate that they represent the sub-aqueous
portion OF the delta of the Potomac when the
The altitude of the terraces is about so feet but
since the Washington deposit extends little
higher 1t Is probable that the old estuary was
never completely filled with sediments, and
that the terrace was fashioned in part during "a
temporary halt in the recession of the sea.
THE CONDUIT TERRACE.
The next notable terrace is the rock shelf
skirting the north shore of the Potomac from
afn,r f<HE
conduit through which the city water supply
is conveyed. 1t is formed Of rock, thinly COW
cred with a mantle OF debris in part derived
from the secular disintegration of the terrane
and In part formed Of the detritus washed river
ward from the adjacent hill. 1t is q terrace of
destruction. and since there are no barriers to
the seward competent to arrest tie course of
the Potomac, 1t obviously represents an old
t pase level me oid base level plain, of which
fectly represented by the Indefinite remnants
IN and north OF LIE Droit park, and by a narrow.
shelf skirting the bluffs east aud north of Fort
The next important terrace is that forming
the highlands IN the north of the city,. The
traveller upon Columbia road, 10th street road
l4th street road, and It'll street road cannot
fail to observe that all OF these roads occupy a
corraded and lObed marginaily by rayines and
abruptly cut Off on the south by the depression
upon which Tsn tne greater part Of the city Is To-
the opposite side OF the river by the level tract
upon which Fort Myer and Arlington stand
The same terrace extends southwark to near
Alexandria. These terraces are manifestly due
to destructive agencies, since in the artificial
cuttings ON 10Th street and 14th street, and
particularly In the projected streets intersect-
ing It'll street road just west OF the Scheutzen
park, the unmodified sands and gravels of the
Potomac formation are seen to form at the sur
face. The terraces Of this level are about 900
feet in altitude.
Tns nss terrace s Is mperey
reaches q great extent on the south side, Its