Year: 1867

CurfOsity of IOwa--Interesting


Letter from the State GeOlogiss.


Iowa CRY, December 9.
To the Editor Of the Dubuque Herald:


From time to time, during the past ten
or fifteen years, the public have been
treated to accounts of the so called
walled. lakes of Northern Iowa, one of
them being situated in Wright and the
other in Sac county, and almost every
writer seems to have entertained the be-
lief that the "walls" were the work Of
human hands, and those were the hands
of departed race of men who, ages ago,
inhabited that region. While making
examinations of the peat marshes of that
part of the State during the past season,
/ had excellent opportunities to examine
both of the lakes just named.


The walls or, more properly, en-
bankments, are really very interesting
natural objects, and it is not surprising
that they have attracted some attention.
They vary much in height and width, as
we'll as in the materials which compose |
them. Sometimes they are principally |
of boulders, but more often of sand,
gravel and earthly material thrown out
of the bed of the lake. In many in
stances where peat marsh extends out
like an arm of the lake, it is entirely
separated from it by an embankment of
turf thrown up by the same agency, but
of turf, because that, and no other ma
terial, was within reach of the ice.
These turf embankments sometimes
4 &c
and have been called beaver dams: but
beavers never attempt to dam still water.
These turf embankments very much re-
semble the material thrown out of a
ditch in draining marsh, but their ori
| gin is unmistakable. When the
embankments are composed princi-
pally of boulders, they are usually
thrown up from two to four feet
high, and from five to fifteen feet wide,
and imbedded in sand. gravel and earth,
the outside of the elnbankment being
usually as steepas the inner or lake side;
and the latter often family resembles an
artificial levee. Although they some
times have degree of regularity, the
boulders which compose them are never
arranged in any order, nor is there an
appearance of any work of art upon
them.


The embankments always separate a
piece of low ground from the lake, but
when the adjacent land is higher than
the highest water of the lake, no en-
bankment is formed, but the materials
are simply crowded against the shore
until bold, steep bank. often covered
with boulders, is produced.


When they are composed principally
of sand and gravel broadly rounded
ridge is formed such as we would expect
from those materials but when composed
of more earthly materials they are often
livery steep and very narrow, and sup
ported by a growth of vegetation upon
them. Such as these were seen on the
east side of Rice lake, where they are in
some instances SO narrow that two per-
sons cannot walk abreast upon them, al
though they are from four to six feet
thigh. In other places they are often


wide enough for wagon, but never suf'
Efficiently even for one. - -


The water in these lakes is almost


always low in the latter part of the


year, and the frosts of winter still furth-


er reduce the actual depth. so that very
little unfrozen water remains in some Of
them. This is often known to be the


case, and only few winters ago nearly
: all the fish of Wall lake, in Wright
county, were killed by that means.


| 1t is evident that whenever the ice


became frozen to the bottom of the lake
! it would freeze fast to, and in many in
q stances inclose the boulders and grave


which were strewed upon the bottom
p Now, when spring returned, the ice be
t ing raised by the rains and melting
! SHOWS, would be carried with its bur


dens to the high-water shore by the pre-
;. vailing wind. Let this process be re
peated year after year, from age to age
and it is evident that all the boulder
L within reach of the ice would in time b
J taken up and carried to the shore, an,
i left exactly where the force of the ic
!, ceased to act. Add to this the almos
. constant dashing of the waves agains
: the beach during the warmer parts G
the year, would have the effect of carry
d ing out large quantities of gravel an
D sand, which would completely imbed th
!. boulders. There is also another cause
:; which doubtless assisted more than an
b other in giving the embankments thei
i. definite form.
II The whole surface of these lake
Of freezes up almost simultaneously, an
ssss to considerable depth. Now the ma
BY ural expansion of solid cake of ic
to from half mile to five miles in diam
"a "a ter, has, as every one knows, enormol
9 power, quite equal to any amount r
- quired to throw up any and all tt


boulders we find in the embankments,
na crowd them quickly against the steep


them back again, and the annual repei
ton of the forces above referred


affords sufficient explanation of the
- phenomena,
! 1t may be thought by some that t
: process described would be too slow


produce the results which we see, b


slowness is quite in keeping with t


mightiest operations of nature. az
/. mills grind slowly, but they grind e


ceedingly fine"


Il The shores of Crystal Lake show t,
I sets of embankments. showing that, Al
! remote period, the lake occupied a high
tc level, ard that its surface was lower


by the deepening of its outlet, when t
, second embankment was formed.


Seeing, then, that the origin of the


embankments can be accounted for
}. the action of natural force alone, it


difficult to understand how any one col


suppose the human hand had anythi


to do with their construction. There


a.,