Burlington weekly free press. [volume] (Burlington, Vt.) 1866-1928, 1915-12-16
LETTERS FROM THE PEOPLE
Large Block of Marble Recently Placed
Near the University Museum
showing Water Erosion.
To the Editor Of the Free Press:

10 tHe fSu1tOr OI tnc FTec
A splendid illustration of the erosion
caused by a stream of water has lately
been presented to the University mu-
scum by the Vermont Marble company
through the thoughtfulness Of Morti-
mer Proctor.
More than a year ago Mr.. Proctor
called the attention of the writer to a
pothole in the Riverside quarry at Proc
tor and offered to hav a rock taken
out when the place should be reached
in the channelling and sent to the mu-
scum.
In due time this was done and this
unique and very interesting specimen is
now located in front OF the Museum
building, it's size and weight making 1t
impossible to place it inside.
The surface of the Riverside quarry
when first uncovered by removing a
layer Of gravel was very greatly fur
rowed by glacial action. During the
latter part of the glacial period the ice
mass had evidently ridden rough shod
over the marble ledge and left it in deep
and large grooves and ridges, some of
them several feet deep or high. At this
time, or a little later, the wearing shown
in the specimen sent to the museum took
place.
Potholes are common wherever a rapid
stream flows over a rock bed. but l do
not think that this was made in this
way exactly. 1t is rather what has
been called a moulin-a glacier mill.
over the surface of most glaciers there
are seen, at any rate during the warmer
months. many streams which make by
their flow deep channels in the clear
ice and as there are always crevasses.
cracks in the ice of the glacier, which
may be hundreds Of feet deep and many
feet wide, sooner or later these streams
fall into one of these and dashing over
the precipice Of ice escape beneath the
great ice stream.
Such a waterfall, even If not very
large, would necessarily wear out a
basin in the surface upon which 1t
strikes and the sand and stones car
ried down with the stream would form
with it a powerful tool which would
cut and wear the stone and excavate a
well or pothole.
AS soft stone is soon ground to pow
der, boulders whenever found are of
hard stone, much harder than marble
and as soon as the first formed depres-
VIVA '"e "OAi wv &icoSh W lOi
within it those boulders that had been
brought down by the glacial stream the
force Of the water would whirl them
around and around and they would thus
themselves be rounded and would wear
out in a softer stone a continually deeper
well. OF course his ~excavating would
be rapid or slow as the ledge was hard
or soft stone.
All n~tlz ..- -: Lt-
but most that We find are the result OF
permanent streams, some. however, are
as was this, formed by streams from
the melting of glacial ice.
1t may be noticed in passing that the
so-called boulder" in front OF the ole
college building is not an ordinary
boulder, but a boulder which has beer
revolved for a long time in some large
pothole, hence its perfectly spherical
form.
Two blocks were taken out from the
quarry. One, a very irregular piece
over seven feet on each side. was or
the top and shows splendidly the gen.
eral erosion Of the surface and the be.
ginning Of the moulin. or glacial pot.
hole, the other. a nearly cubical block
over five feet on each side, contains the
main cavity. which is so inches deer
and 40 across the top. The two blocks
are said to weigh IS tons.
This cavity. and indeed the whole
mass. being from a quarry of finely
veined white marble, is unusually attrac.
tive aside from its interest as a.speci.
men of erosion.
very much doubt if anywhere there
is another such example. though there
are many similar in coarser material
usually in granite or some such rock.
G. II. PERKINS.