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

the Potomac nor any other considerable stream
could have reached the tract without greater
cal: be postulated, and since, too, no such
stream could have fashioned the plain without
it is manifest that this is beach terrace. The
tide.
OTHER TERRACES.
Higher terraces occur on both side Of the PO-
tomac. Some of these terraces are represented
by the plain between Georgetown and Tenally-
town; the plain upon which Woodley road is To-
Gated; by the long spur southwestward from
,Ienallytown: by the highest portion Of the Sol
the Reform school; by the plain south of the
chain en-, bridge; by the spurs encroaehing upon
etc; but they are SO widely scattered, and SO
greatly modified by erosion, that without &C.
curate maps they cannot be successfully CORE
The eastern shore of the Potomac and the
Eastern Branch are as conspicuously terraced
Insane stands upon the margin Of a terrace
plain, SO broad and SO nearly horizontal that
its interior is imperiectly drained. 1t is occupied
in part by the old national race course and
tournament grounds. This terrace is 100 feet
in altitude, and extends six miles southwark,
and the Potomac. This terrace is overlooked
from the north from tue scarcely less definite
and horizontal plain traversed by the Hamil,
Hope. 1t extends several miles further north
ward, and in that direction is deeply incised
the eastward beyond sA oxen S' R spp,A's and
SstEssp tbs ss s5@saa and %sn!sas fs
it has been found impossible to correlate this
and other terraces on the east side of the Poto-
in determination Of attitudes being OF greater
value than the vertical distances separating
the different terraces.
TERRACES OF THE THREE, MOUNTAIN DIv1-
Ascending the Potomac the first terrace plain
OF considerable area is found on the south side
OF the river between Leesburg and Whites
Ferry. Its altitude is about sixty feet above
the river. 1t has slight declivity in the direc-
is red sandstone, and schists OF the Triassic
ana Azolc groups, 1T bears upon its surface
thin matter of pnlverized loan, evidently allu-
vial In origin. Thus 1t appears to be o terrace
of PLANTATION. Less conspicuous terraces occur
at attitudes up to about 200 feet above the
river in different terraces of the plain section of
the Piedmont division, but they are in general
Indefinite and cannot be correlated in either
their vertical or horizontal relations without
In existence. Most of them appear to be ter
races OF PIanat1On; some of the highest plains
are made up of coarse detritus, iithologfcaiy
formation.
The conspicuous plains of the Trans-montane
division OF the Potomac valley are those of
Loudon, Pleasant and Shenandoah valleys be-
fore described. In tar as they can be re-
garded as terraces they are maniiestly terraces
OF destruction; and the entire absence of super.
fetal detritus argues their base level origin.
In addition to the old base level plains of the
bly, terraced and third from which terraces
are as notably absent. In Green Spring valley
basin there Is absolutely no terracing in the
valley sides. There is no considerable accumu-
lation OF alluvial or other superficial detritus,
and the river flows throughout in channel
f11n tn
above the river. Their escarpments expose
siratified sands and ciays, with occasional in
of local aud sub-Iocal origin. They are mani
testify terraces of construction, and the amount
OF accumulated material is such Is to indicate
that the entire basin was at one time filled with
alluviai or possibly iacustral deposits up to the
level OF the highest terrace. The terraces of
Moorfield basin are of like character aud gene
sis, though the materials of which they are
composed are finely Illuminated clays, with In
tercuia;ed layers of cross-laminated sands, the
The condition Of the materials is such Us to sug-
get a lacuStrai origin, though it is perhaps
broad, wandering river OF little declivity.
THE SIGNIFICANCE OF THE TERRACES.
The obvious genesis of the Washington brick
clays and gravels indicates that at q period
probably as recent as the late Quaternary the
sea occupied the broad basin in which Wash
ington is located; the PROFESSIONALLY base-level
from Georgetown to Great Falls proves that at
period whose remoteness is measured only by
6a de r o. 'he o marrow at gorge below the
level; the phenomena of the great terrace-plain
extending from Fort Ayer to Falls Church
subsequent erosion has not yet extended itself
over the entire plain, the oceans waves washed
a. shore $50 feet higher than that of to-day; and
the higher terraces at TenleytOwn and else.
where suggest that beach action took place
more than 400 feet above its present limits ut q
date SD recent that the beach marks yet remain
upon the ephemeral hills. In like manner the
Old baSe-leTel plains In the montane divisions
OF the valley are indicative of terrestrial eleva-
zion at an epoch SO near the geologic to-day
that the stream is yet struggling to adjust itself
TO the new relation between land and sea. The
valley liz geoOsically recent time must there-
fore be regarded as established.
me quality OF the elevation is also recorded
by the terraces. Thus far inclination of the
beach-iormed terraces in the vicinity of wash
1ogton has not been detected; but the fact that
The corrasion Of the gorge below Great Falls was
initiated suggests that there Is relatively greater
elevation Gle, to the northward and consequent
tlItlns OF the land to the southwark. Moreover
the greater altitude above the river of the old
Dase-levei plains of the montane divisions to
greater relative elevation there by an amount
measured by this difference in altitude plus the
SIN Sill greater amount required TO reduce the
flowed at base level, while the new streams flow
far higher than their base level But the differ
simple fitting. The difference in altitude of neigh
boring lnter-Elontane plains in the Appalachian
zone snggests that contiguous geographic blocks
nave moved independently since the base-level
at the Blue Ridge, whe1 the broad shenandoah
eastern margin lifted above the level Of the
adjacent plain of Pleasant valley. It Is still
m9fe decsiysly indicated in the syn
through range remained free from sediments, |
while the basin drained westward through the |
same range became filled, and by the basin |
drained through the axis of q range In which |