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

TESTIMONY OF THE TERRACES.
| How the Potomac Valley has been
Sculptnrcd by Time and Tide.
EVIDENCES THAT THE SEA ROLLED UPON THE
HILLS ABOUT WASHINGTON IN RECENT GEO
LOGIC TIMEs-THE GEOLOGY OF THE POTOMAC
| VALLEY-A RIVER AS OLD AS THE NOUN
TAINS THROUGH WHICH IT MEANDERS-
SYNOPSIS OF AN INTERESTING PAPER BY MIR.
w. j. M'GEE.
At the last meeting of the Philosophical So
ciety Mr.. w. ]. MCGEE, of the geological survey,
who has explored the whole extent of the Poto-
mac valley, read paper on "The Terraces of
the Potomac Valley" which excited much in
terest He Illustrated the subject matter with
a large chart of the Potomac valley, Of which
| the map accompanying this article is a reduced
| fac smile.
Like other rivers, Mr.. MCGEE said, the Poto-
| mac has made its own bed; but the process has
| not progressed uniformly, and its irregularities
have recorded themselves in the terraces which
skirt the valley from origin to embouchure.
These terraces tell of variations in relative level
| OF land and sea, and with their correlat1vc de-
mOdic and ditferential elevations of the Ian d.
commencing in the later Mesozoic, progressing
IL
| out the Tertiary, and eliminating in Quaten-
airy and recent time.
| TO render the testimony OF the terraces in
teillglDle, MI. McGee gave an analysis of ter
| races applicable to those of the Potomac valley,
and description of the geography and geology
of that valley. He separates the terraces of the
Potomac as well as those of other similarly Sit
uated rivers of the Atlantic slope, into two cat
! egorles defined by the general mode of origin,
| VIZ: l.) Terraces of construction and (2) ter
races OF destruction. Terraces of construction
are formed of alluvium deposited by streams
waves ana currents of oceans. streams and
| rivers. Terraces of destruction are carved out
OF pre-eAisting terrane by wandering streams
and by the waves and currents of oceans and
estuaries. The same terraces may be again
| separated into two classes, defined by specific
mode of operation of the agencies by which they
were fashioned-Q) beach terraces and (2) ter
races of PLANTATION. Beach terraces are fash
| Toned by oceanic estuarne, and lacustral waves
| ana currents, may be either constructive de-
structive, and are pre-emnently characterized
by horizontality. Terraces of PLANTATION result
which either from the proximity of base level
| or the presence of BARRIERS are incompetent to
corrode vertically. They may be separated into
| two sub-casses-base level terraces and barrier
terraces. Both base level and barrier terraces
are characterized by a gentle inclination, cor
responding to the declivity of the stream by
which they were formed. A notable departure
! from such gentle inclination in ancient terraces
Is indicative of ditferental elevation. Absolute
level particularly in the case OF base level ter
races Is Indicated by the elevation of the ter
| race above the stream.
GEOGRAPHY OF THE POTOMAC VALLEY.
| Regarding the longest affluent as the river
proper, the Potomac and its valley are natu-
rally separated into four divisions which, Mr.
MCGEE stated, may be denomina:ed respec-
| lively: 1, the synmOntane division, extending
from the source of the river to the confluence Of
| sion. extending from the confluence to the
easternmost range OF the Blue Ridge system
extending from the Catoctin to the head of tide,
water at Georgetown; 4, the tide-water division,
comprising the estuarine portion of the river,
extending from Georgetown to Chesapeake
| bay.
Throughout the synmontane division the river
generally occupies valleys separating the par
alel Appalachian ranges, but it occasionally
breaks through one or more of the ranges
Thus the immediate river valley is q success sion
Three Of these basins-Moorfield basin, Rom
ney basin and Green Spring valey-were de-
seribed in detail: A striking feature of the
division IS the existence of Il double reiief-a
| high relief expressed by the mountain ranges
and the main river valleys, and a low relief
expressed by the foothills forming the imme-
| date river bluffs and the ravineS of the sec
ondary and tertiary tributaries. The crests OF
tSs as a9ers6K h and the slopes
old mountainous regions. The slope from
| the crest OF the range to the principal water
and progressively diminishing slope to the
waterway, and again so steep as to form some
times precipitous escarpment whose base is
washed by the river. Another notable feature
of the division is the rocky character of the
river bed. Fully nine-tenths of the
rock, and nearly all OF the remaining portion of
the bed is made up of great bouldars and ang,l-
lar blocks manifestiy little removed from their
parent lodges, which churn the river into u
raging torrent. MIR. McCiee infers from the fact
that the river valley, though 1t generally fol-
lows, sometimes deserts the depressed cOrruga-
tions or geographic valleys, that the river is
about of equal age with the mountains among
regions OF active Notwithstanding have shown
that rivers maintain their courses despite
the orogentc movements.
THE TRANSFORMING DIVISION.
In the transmontane division of its valley the
Potomac battles with the Appalachians, and
though compelled to meander OBVIOUSLY to
right and left by successive ridges of obdurare
rock, 1t generally cuts directly through the giant
corrugations constituting the Appalachian
mountain system, breaking through the success
ogous to the water Gaps" of the Delaware and
the Susquehanna, and a like number of trans
verse ;,, t,roughS, sometimes expanded into con-
andOah plains-the most remark able interlnon-
tame plains of the transmontane division
together with the less conspicuous plains TO the
| westward are unmistakable expressions of the
double relief less strikingly manifested in the
synmontane division; The transmontane river
bed Is as notably rocky as is that Of the proXi-
NAL division, the river flowing for fully four
fifths of It's length upon undisturbed lodges of
| NTH upon shifting beds of boulders and coarse
gravel and sharp sand. Thus the entire cham
mel Is LuALl!eStiy undergoing degradation.
THE PIEDMONT DIVISION
| OF the Potomac valley comprises a gently undu-
lating plain to the westward, extending from
the Catoctin range, at the Point of Rocks to the
| first gneissic range of the Washington belt at
Seneca, corresponding In a general way with
the triassic terrane, and low, iil-defined pla-
| teau to the eastward, extending from Seneca to
Georgetown, and corresponding with what has
usually been regarded as the Washington zone
| OF gneiss The gorge occupied by the Potomac
| traverses the plateau. It is sometimes i'll
| defined lateral.y, but save where modified by
| recent erosion, remarkably uniform in the con-
Potomac flows upon this old valley bottom
which IL has almost entirely denuded of debris
| to Great Falls, where 1t enters the newer and
narrower gorge which it has excayated for
itself in the south side Of the old leaving half
Of the old valley bottom as a continuous shelf
overlooking the present channel from height
measuring from twenty feet Immediately below
1 the falls to one hundred and fifty feet at the
distributing reservoir near Georgetown Save
where II Is affected by artificial Structures, the
whole Of this portion OF the Potomac flows upon
ing active degradation, particularly ut the locus
Of great ACTIVITY at Great Falls and the locus of
less ACTIVITY at Little Falls.
/ The Underwater division of the river Is an ca.
tuary OF considerable though variable width
| In which sedimentation is unQuestionably in
/ active progress. 1t Is hanl1ed on both sides bv
terraced shores rising In successive steps