Page images
PDF
EPUB
[ocr errors]

CALIFORNIA.

it gives not only the general direction of

movement of the glacier, but its local variations. Sometimes two sets of striæ can be seen running in different directions, one of them crossing and hence later than the other. These usually mark the moveinent of a later glacier, for we know that the ice which covered the country advanced and retreated no less than five times. The evidence of these successive stages can be sought to better advantage, however, in the topography of a region than in the markings on the rock surfaces. It is evident that the effect of the movement of such a vast mass of ice and rock over a region must have been

to plane down its prominences and smooth GLACIATED MOUNTAIN PEAKS, TUOLUMNE VALLEY,

and round over the whole area. This smoothing and rounding effect is well shown

in the mountain prominences seen in the acany one time their full significance. We

Here the glaciation has now know, however, that the shapes of the companying cut. pebbles and the scratches upon them indicate

been so recent that vegetation and decay

have not come in and obscured the surface glacial action, and that where we stand amid

as is the case over most of the area of the a landscape clothed in the verdure of sum

northeastern United States. Contrast this mer, but a short time ago, geologically with the high bluffs along the Mississippi speaking, winter, centuries if not millenni

river in Wisconsin, which are in a nonums long, reigned and wrought. So just as the pearly shell held to the ear repeats the glaciated area, or with

pinnacles such as

there are many of in Colorado and other story of its ocean home, these faceted pebbles could tell us if we listened of the march

Western States, the result of long erosion of a vast sheet of ice by which they were

untouched by glacial action and the effect of caught up and swept onward, and in which glacial movement can readily be seen. they were held and rubbed, now against

If now one thinks for a moment what their fellows and now against the rocky floor beneath, until they acquired the shape

We know this be-
cause we know that it is just what glaciers
do to the rocks they carry nowadays, and no
other force can quite imitate their effects.
Having thus once become aware of the

Formato
former existence of a glacier in the place
where we are standing, the interpretation of
every feature around us will be influenced
by that fact. If along the roadside we find
a place where the blanket of soil has been re-
cently removed so as to expose the hard
ledge beneath, the surface of the latter will
be seen to be smoothed and leveled, and in
occasional exposures, scratches or grooves
somewhat like those on the pebbles just
spoken of except that they are deeper, more
regular and nearly all run in one direction,
may be seen. These clearly mark the plan-
ing down by the glacier of the surface of the
rock over which it passed and the direction
of movement. The recording, according to
the direction of the compass, of these striæ
in several places in any given locality is al-
ways a work worthy to be performed, since

[graphic]

which they now possess.

[ocr errors]
[ocr errors]

WEET

ORICTLESS ANCA

TERMINAL MORAINE

AREA IN NORTH AMERICA COVERED BY CONTINENTAL

GLACIER.

further results we are one cartaceea the glacier: one the existence over the com os co med lase Sezervor lying north tinental i sherbe * Sards 22 visoschise ice that there an si carti aur

*35be asse card and the other hare betin againsta

os be regica caused the monceasi andrior ::

Citizin which received to it was crea var.

Cirsi be deposition of The Deeting of the glavese agencies would

Scociis gravity, the Ni ses of the glacier

IT Sears of water in KITA wcis det and disin a they could

ce Iciies would evi- 2 size the lay 3 Iunie vibe glacier

ca Tesbici expect,

sire. as i large boules 2 is some 12. Die broad,

Des ne soil, SEZ-a dates sature,

[graphic]
[ocr errors]
[ocr errors]

Tacations Sre : sese serial

[ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][merged small]
[graphic][ocr errors][ocr errors][subsumed]
[graphic]

1674

the swift rushing waters are dropped as
soon as the slope and speed diminish and
how in quiet pools or where the flow is
sluggish, even the fine silt can be held no
longer and is laid down. He should note
how at any particular point deposition vary-
ing from coarse to fine resulting from chang-
ing speeds of the current produces a succes-
sion of layers, which, known as strata and
preserved when the whole is consolidated
into rock, would show the nature of their or-
igin. He should note how the cutting power
of the stream is exerted rearwards as well as
laterally, and that the backward movement
tends to subdivide constantly into new
branches so as to give the valleys an amphi-
theater like shape and keep the slope con-
stantly steep. He should note with what
unerring touch the stream distinguishes the
hard strata or veins from the soft and cuts
away the latter while leaving the former
standing in relief. He should note how in
its lower course where the slope is less
steep, the stream is choked by the deposi-
tion of material which it can no longer
carry, the valley is broadened and the
stream winds in it from side to side. He
should note how with a diminution in the

[graphic]
[ocr errors]

BACKWARD CUTTING BY STREAM. EACH ROUNDED
POOL MARKS LOCATION OF A PREVIOUS FALL.

WATKIN'S GLEN, NEW YORK.

of the Middle West. One at Lemont, Illinois, is shown on page 1668.

It is evident that the flowing waters from the glacier and the subsequent continuous drainage of a region will have another effect besides that of mere deposition. They will carve and shape the deposits already made. Since such carving by running water has been one of the most effective agents in forming the striking features of a landscape, one should study it thoroughly. One should observe a running brook or even the rills from a recent rainfall and see what a constant carrier running water is, always moving rock and soil nearer the

He should note how the size of the particles carried varies with the speed of the water, how the large pebbles borne by

sea.

THE PEBBLES OF A BROOK BED ARE SORTED AC-
CORDING TO THE RATE OF FLOW OF THE WATER.

(Photo by H. Wm. Menke.)

[graphic][ocr errors][subsumed][subsumed][subsumed][ocr errors][ocr errors]
« PreviousContinue »