the west end of the section is more thoroughly ground
up and contains smaller and fewer boulders. Between
Wyanet and Loceyville stratification is a common feat-
ure in the boulder clay, and quite often the material is
seen to be cemented together so as to form a solid
mortar rock, the cementing material being in most
cases carbonate of lime, often quite pure but sometimes
ferruginous. The mortar rock is known by well-drillers
as "hard-pan." In the Bureau creek valley the lower
part of the drift is composed of sand and gravel.
Below the boulder clay and its assorted materials
there has been found in some borings a fine, bluish or
reddish silt, somewhat like loess in appearance, and
resting on the bed-rock.
The Coal Measures.
The highest beds in the coal measures series are seen
at LaSalle, north of the city. They consist of variously
colored shales, alternating with thin seams of limestone
and carbonaceous material. A section taken at this
place is as follows:
129
SECTION AT LA8ALLE.
A. T.
(1). 37 feet variously colored shales 580-543
(2). 1 foot blue limestone (weathering rod) 543-542
(3). 2 feet bituminous shale and coal 542-540
(4). 32 " variously colored shales 540-508
(5). 20 " limestone 508-488
(6). 3 " bituminous shale and coal 488-485
(7). flre clay (sometimes absent). ..
(8). 17 " shale 485-168
(9). 2 " limestone (several feet concealed) 468-466
(10). shales
NOTE: In the following pages these numbers refer to the eleva-
tion above the level of the sea in feet, a minus sign ( ) being used
when the section is below the sea level. The levels were obtained
by aneroid measurements checked to the nearest known level.
In the bluff at Locey ville there are exposed some beds
which may probably belong to a lower horizon. A sec-
tion of the greater part of the bluff at this place is as
follows:
SECTION AT LOCEYVILLE.
A. T.
,(1). 7 feet limestone 527-520
|(2). 26 " bluish shale 520-194
(3). 2 " limestone 494-492
(4). shale (mostly concealed)
About a mile west of Tiskilwa in the Rocky Run, the
following exposure is seen:
SECTION IN ROCKY RUN, TISKILWA.
A. T.
(1). 40 feet sandy shales 674-634
(2). 4 " shales with nodular seams of ferrugin-
ous limestone 634-630
(3). 2 " coal 630-628
(4). Probably several feet of limestone
-9
130
A limestone which cannot be far below the base of
this section is broken up into huge blocks which are
seen in the bottom of the creek. This limestone is prob-
ably identical with the upper limestone in the previous
section.
Near Sheffield the following section is seen:
SECTION IN COAL, CREEK, SHEFFIELD.
A. T.
(1). 12 feet micaceous sandstone thin bedded, often
rippled-marked, with ferruginous no-
dules, indurated below 683-671
(2). 15 " micaceous sandy sbales above, argillace-
ous shales with bands of nodules below 671-656
(3). 1 foot black indurated thin splitting shale 656-655
(4). 5 " coal 655-650
(5). 2 " limestone 650-648
Numbers 4 and 5 in this section are the equivalents of
3 and 4 in the previous section. The same succes-
sion is seen south of Mineral, where Mr. W. H. Forrest
has sunk a shaft and observed the following section:
MINERAL.
A. T.
(1}. 12 feet sandstone 709-697
(2). 18 i: light colored shale 697-679
(3). 6 " bituminous shale 679-673
(4). 5 " coal 673-668
(5). 1 foot flre clay 668-667
(6). Limestone
The beds which are found under the limestone of these
sections are not exposed anywhere along the line of the
survey, except in the vicinity of LaSalle, but from borings
which have been made in various places in Bureau and
LaSalle counties they are known to consist of alterna-
tions of shales and sandstone, with seams of coal and
limestone. In a well which was bored about a mile and
131
one-half southpastof Annawan, on Mr. L. A. Ferguson's
farm, strata below this horizon were passed through as
follows :
ANNAWAN.
A. T.
(1). Drift
(2). 6 feet miner's "slate" 557-551
(3). 3 " coal 551-548
(4). 1 foot fireclay , 548-547
(5). 10 feet shale 547-537
(6). seam of limestone
(7j. 16 " sandy shales 537-521
At Atkinson coal is mined at a small depth, from a
seam which is probably identical with number 3 in the
section of Ferguson's well. In Mr. Riley's mine, south
of the city, the succession of the beds is as follows:
MR. RILEY'S SHAFT, ATKINSON.
A. T.
(1). Drift
(2). 4 feet shale 648-644
(3). 1 foot concretionary limestone 644-643
(4). 15 feet shale 643-628
(5). 1 foot limestone 628-627
(6). 2 feet shale 627-625
(7). 3 " miner's "slate" 625-622
(8). 3 " coal 622-619
(9). Fire clay
The same is seen in a shaft on Mr. James Kay's farm,
four miles west of the city.
MR. KAY'S SHAFT, ATKINSON.
(1). Drift
(2). 1 foot limestone 642-641
(3). 17 feet shale 641-624
(4). 2 " miner's "slate" 624-622
(5). 3 " coal 622-619
132
Mr. Johii Mowbray some years ago explored below
this coal not far from Mr. Kay's place and found the
following beds below it:
MR. MOWBRAY'S SHAFT, ATKINSON.
A. T.
(1). 4 feet pyritiferous flre clay 619-615
(2). 2 " concretionary limestone 615-613
(3). 12 " blue shale 613-601
(4). 2 " sandstone 601-599
(5). 1 foot coal 599-598
(6). 4 feet fine flre clay 598-594
(7). Impure flre clay -
The lower part of the coal measures contain a greater
percentage of arenaceous material. This is first seen at
Geneseo.
SECTION IN GENESEO CREEK.
A.T.
(1). Concretionary
(2). Sandstone (partly concealed) -
(3). 2 feet black shale 623-621
(4). 1 foot coal and shale 621-620
(5). 3 feet flre clay 620-617
It is also seen in the ravines back of Carbon Cliff.
SECTION OF ARGILLO WORKS CLAY PIT.
A T
(1). Drift . 1_L
(2). 10 feet sandstone 645-635
(3). 1 foot coal 635-634
(4). 4 feet flre clay 634-630 .
(5). 25 " shale 630-605
In Heagey's mine, south of Port Byron Junction, the
succession is as follows:
HKAGEY'S MINE, PORT BYRON JUNCTION.
A T
(1). 50 feet drift 684-634
(2). 14 " sandstone and "cap rock" 634-620
(3). 3 " coal . 620 _ 617
133
Donald and Jamison's shaft, about a mile southwest
of this place, was sunk through the following strata:
DONALD AND JAMISON'S SHAFT.
A. T.
(1). Drift
(2). 15 feet sandstone 630-615
(3). 8 " dark shale 615-607
(4). 2 " coal ("bone coal") 607-605
(5). 4 " fireclay 605-601
(6). Not reported 601-590
(7). Coal -
The arenaceous character of the lower part of the
coal measures is, however, best developed east of Ot-
tawa. At Marseilles and Seneca there is a sandstone
over 50 feet thick, and in the vicinity of Morris the ex-
posures of the coal measures along the canal mostly
consist of sandstone. This is the equivalent of the
famous fossil bearing sandstone of Mazon creek.
The character of the material constituting the base of
the coal measures is quite varied. Most commonly it
consists of sandstone. This is the case farthest east in
the DuPage river south of Channahon in the Kankakee
river above its mouth, and in Au Sable creek, east of
Morris, West of Morris the lowermost of the coal meas-
ures are composed of shale overlying a seam of coal, as
seen in the following sections:
PRENDERGAST AND M'CLARY SHAFT, SENECA.
A. T.
(1). Alluvium
(2). 20 feet sandstone 502-482
(3). 8 " "soapstone" 482-474
(4). 2 " hard black shale 474-472
(5). 10 " sandstone 472-462
(6). 63 " "soapstone" 462-399
(7). 3 " coal ... 399-396
134
CARNEY BROTHERS' SHAFT, % MILE EAST OF MARSEILLES.
A. T.
(1). Drift.
2 . 4 feet coal 551-547
3 . 57 ' sandy shales and sandstones 547-490
4 . 1 foot black miner's slate 490-489
(5). 69 feet shales 489-420
(6). 3 " coal 420-417
RIVER BLUFF, MARSEILLES.
A. T.
(1). Drift
(2). 60 feet sandstone 580-520
(3). 5 " dark shale 520-515
West of Ottawa, the coal measures may be said to be-
gin with a seam of coal resting on the St. Peter sand-
stone. At Split Rock they consist, as already noted, of
brecciated fragmentary material. In the vicinity of
Bureau Junction, where a number of borings have been
made, some of the drillers have reported "zinc ore"
from the horizon under consideration, but as no com-
petent analyist has examined the material, so far as
known, nothing definite can be stated as to the nature
of the borings so named.
At Annawan the lowest strata of the coal measures
run as follows:
SECTION OF ANNAWAN CREAMERY WELL.
A. T.
(1). 124 feet drift 629-505
(2). 14 feet dark shale 505-491
(3). 1 foot bituminous seam 491-490
(4). 21 feet shale 490-469
(5). 1 foot limestone 469-468
(6). 1 " coal 468-467
(7). 1 foot flre clay 467-466
(8). Niagara Limestone.. .
135
On the lowlands along Green river coal has been
stripped from the top of the Devonian limestone, and
this coal was capped by a black bituminous limestone,
but some distance to the west of this place a sandstone
is found occupying the position of this coal. Along the
Mississippi river, near Port Byron Junction, the little
remnant left of the coal measures consists of sandstone,
in which pebbles are occasionally found imbedded.
In the city of Rock Island an outlier of Devonian
limestone is capped by a small remnant of a hard con-
glomerate, which consists for the greater part of chunks
of yellow chert containing Silurian fossils. Though
positive proofs are wanting, there are good reasons to
believe that this conglomerate belongs to the coal meas-
ures.
'
A Marked Unconformity.
The coal measures are separated from the rocks below
them by a marked unconformity. For the greater part of
the distance across the State it appears as an uncon-
formity without tilting. At Rock Island it displays
erosion forms in the underlying horizontal strata, with
reliefs of nearly 70 feet, and the lower rocks are studded
with caves which are filled with sandstones and shales
of the coal measures. Similar appearances have already
been referred to above as occurring near East Moline
and at Carbon Cliff. In nearly all of these localities the
underlying limestone is studded with caves which are
filled with sandstones and shales belonging to the coal
period. South of Channahon, on the bank of the Du
Page river, eroded depressions in the Niagara and the
Cincinnati formations were observed to be filled with
coal measure sandstones, in which were found, with im-
perfectly preserved woody tissue of plants of the coal
age, various fragmentary materials of the underlying
136
rock, such as lumps of the Cincinnati shale and small
blocks of Niagara limestone, containing casts of the
fossils of that age.
At LaSalle it appears as an uncomformity produced
by tilting with a differential precarboniferous erosion of
900 feet, this thickness having been removed (in ex-
cess of the denudation on the west side) from the under-
lying rocks on the east side of the monocline at this
place. In its general aspect, the unconformity may be
said to be connected with a general tilting of the sub-
jacent formations, for these are seen to slowly ascend
and run out under the superimposed coal measures, both
in the east and in the west end of the section.
The Rocks of the Devonian Age.
To recapitulate what is already stated in the descrip-
tion of the superficial features, the exposed Devonian
rocks may be given as follows:
SECTION NEAR ROCK ISLAND DEPOT.
A. T.
(1). 20 feet shaly limestone and calcareous shale,
containing throughout fossil brachio-
pods, and near the top stems of various
crinoids 599-579
(2). 6 " composed of three ledges of limestone
separated by seams of shale, and con-
taining about the same fossils as the
beds above 579-573
(3). 7 " consisting of three solid ledges of a
strong limestone, containing a less num-
ber of brachiopods and more corals 573-566
(4). 5 even grained, sometimes brccciated,
limestone, containing no fossils 566-561
Below this there are probably some 70 or 80 feet of
limestone, like No. 4 of the above section, but though
there are quite a number of exposures of this limestone,
no continuous section can be constructed for this local-
ity. On Sylvan Island, just north of Moline, there is a
typical exposure of these lower beds which may be
described as follows:
SWAN TROPP'S QUARRY, ON SYLVAN ISLAND.
A. T.
(1). 7 feet irregularly bedded white and hard lime-
stone, but little brecciated and weather-
ing into thin layers 577-570
(2). 5 " darkish compact limestone, with a pecu-
liar lamination, occasionally presenting
a low dome-shaped structure 570-565
(3). 1 foot pure and white, massive limestone 565-564
(4). 4 feet dark limestone, resembling No. 2 564-560
(5). 3 " white limestone, sometimes massive, and
sometimes brecciated and composed of
good-sized blocks of dark material im-
bedded in the white 560-557
From borings made at Rock Island it seems as if
there were 40 feet or more of similar rocks below this
section.
Materials resembling these rocks are reported from
the boring at Geneseo, but nothing can be said with
certainty as to the occurrence of Devonian rocks at this
place. In the boring at Annawan they are, without a
doubt, absent, but in several borings in Bureau creek
valley and in the artesian well at Hennepin, as well as
in the wells at Peru and LaSalle, calcareous shaly beds
were encountered on top of the Silurian limestone, which
in all probability belong to this age; but here again
positive knowledge from fossils is wanting. The thick-
ness of these undetermined shales averages 100 feet.
138
THICKNESS AND POSITION OF DEVONIAN ROCKS.
(Estimated from borings).
A. T.
Rock Island, shales and limestone ... 80 feet. 599 519
Moline, limestone 50 " 565515
Carbon Cliff, limestone 80 " 580 500
Searles' Well, limestone (?) 40 " 560520
Geceseo, limestone (?) 40 " 520480
Princeton, shales and limestone (?)100 " 120 20
Hennepin, shales-and limestone (?)100 " 130 30
LaSalle, shales (?)130 " 0130
The Contact Between the Devonian and the Silurian
Systems.
The contact between the Devonian and the Silurian
rocks is not exposed for certain anywhere in Rock Isl-
and county. About one-half mile below Hampton there
is seen in the bank of the Mississippi some buff-colored
limestone, which Professor Worthen refers to as the
southernmost exposure of the Niagara limestone along
the river in this county. (Geol. Surv. of 111., vol. V.,
p. 223.) There is no doubt that the top of the Niagara
limestone comes near to the surface at this place, for it
soon appears in the river above Hampton. But on a
closer examination of the rocks at this point they show
a greater resemblance to the lowest unfossiliferous layers
of the Devonian rocks than to the uppermost of the
Silurian strata. The color is the same as that of the
latter, but this color is due to weathering of an
originally bluish-white limestone, like the Devonian
in texture, as can be seen on a freshly broken sur-
face of some of the ledges. It may be that the rocks
seen farthest to the north at this place belong to the
Silurian system. If such is the case, we have here the
contact between the two systems. Nothing is exhibited
in distinctness. The beds are all apparently horizontal.
But there are found in the limestone broken pieces of
139
chert, which have a decided appearance of being trans-
ported fragments rather than concretions formed in
situ. This is the only evidence of an unconformity be-
tween the two systems at this place.
If the undetermined shales under the coal measures at
Peru and LaSalle belong to the Devonian age, it would
seem that we have at this place another evidence of an
interval of erosion at this horizon. In the well bored
by the Illinois Zinc Company the thickness of the
Niagara limestone is considerably less than in the wells
farther west. This may be best explained as being due
to erosion previous to the deposition of the shales sup-
posed to belong to the Devonian age.
The Silurian System.
The Niagara Limestone. The Niagara limestone forms
the uppermost member in this system. Its thickness in
the west part of the State is only known from borings,
and is found to vary from 275 feet to 400 feet, averag-
ing 350 feet. The upper part seen on the Mississippi and
on Rock river is yellowish and hard, but rendered some-
what porous by the removal of imbedded corals and
stems of crinoids, the empty moulds of which remain.
It is seen to have an oblique and variable bedding,
which persists across the State, being well exhibited in
the exposures northwest of Manhattan in Will county.
Drill cores taken below the coal measures from Mr.
Weise's well in the Bureau valley were from rock belong-
ing to this horizon. The lower part of the Niagara
formation consists of a bluish white, compact, and
evenly bedded limestone which contains, especially in the
upper part, bands of gray and white chert. It was
identified in drillings from the creamery well at Annawan.
140
THICKNESS AND POSITION OF THE NIAGARA LIM3STONE.
(Estimated from borings).
A. T.
Rock Island 364 feet. 519-155
Moline 349 " 515-166
Carbon Cliff 388 " 500-1 12
Searles'Well 370 " 520-150
Geneseo 383 " 480-97
Annawan (eroded) (?)300 " 466-(?)!66
Princeton 335 " 20315
Hennepin 350 " 30320
LaSalle (probably eroded) 234 " 130 364
Joliet (eroded) 230 " 540-310
Chicago (eroded) 254 " 530-276
The Cincinnati Formation. The Cincinnati formation,
which underlies the Niagara, varies in thickness, as re-
ported by drillers, from 68 to 250 feet. The exposures
in the vicinity of the headwaters of the Illinois river
are too much scattered for correlation into a continu-
ous section, but in a series of borings taken with more
than ordinary care, by Mr. W. Moore, from the artesian
well at the Illinois Zinc Company's Works at LaSalle,
the following succession was seen:
A. T.
(1). 16 feet blue shale, occasionally stained with
iron 364 380
(2). 5 " light blue shale 380 335
(3). 25 " blueshale 385410
(4). 5 " gray crystalline limestone 410415
(5). 25 " hard brownish limestone 415440
(6). 10 " blue shale* 440 459
(7). 11 " gray limestone 459470
(8). 5 " bluish limestone 470475
(9). 8 " brownish limestone 475483
(10). 18 " dirty brown limestone .'...483501
(11). 12 " shale and limestone 501 513
(12). 29 " lighter limestone and shale.. .513542
141
Number 4 in the above section resembles a limestone
belonging to the Cincinnati, which is seen around Goose
Lake in Grundy county. It is there crystalline and may
almost be called a marble.
THICKNESS AND POSITION OF THE CINCINNATI FORMATION.
(Estimated from borings).
A. T.
Rock Island, shale 200 feet. 155 45
Molme, shale '. 213 " 166 47
Carbon Cliff, shale 180 " 112 68
Searles' Well, shale, sundy grit below.. 75+ " 150-(?)
Geneseo, shale 95 " 97-2
Princeton, shale with limestone near
middle 175 " 315 490
LaSalle shale, with two bands of lime-
stone near middle 138 ' 364 502
Morris' Driving Park, white shale
(eroded?) 70 " 430-360
Blodget, bituminous, calcareous shale. 75 " 505430
Joliet, shale 68 " 310-242
Chicago, shale, with limestone near
middle .. ...... . 250 " 276-26
The Trenton Limestone. The Trenton limestone, which
follows in downward succession, is quite uniform in its
development, averaging 350 feet in thickness and vary-
ing less than 70 feet either way. In its upper part it is
hard and slightly crystalline and white or brownish in
color, while below it is bluish and massive, sometimes
slightly brecciated and occasionally containing fine
sand. The only exposures along the section have already
been noticed.
142
THICKNESS AND POSITION OF THE TRENTON LIMESTONE.
(Estimated from borings).
A. T.
Rock Island 440 feet. 45 485
Moline 320 " 47367
Carbon Cliff (not bored through) 241+ " 68-(?)
Searl' s' Well, (exact limits unknown,)
at least 250
Geneseo 430 " 2 428
Princeton 410 " 490 900
LaSalle 405 " -502907
Marseilles (eroded), a few feet at 410
(?)Peddicord's Well (eroded), perhaps 20
feet at 390
Seneca (eroded) 120 " 400-280
Hoge's Well (eroded?) 200 " 480-280
Morris' Driving Park (eroded?) 170 " 360-190
Blodget 300 " 430-130
Joliet .333 " 242 91
The St. Peter Sandstone. Below the Trenton forma-
tion lies the St. Peter sandstone, ranging from 140 to
240 feet in thickness and averaging 200 feet. It is
white, friable, pure, siliceous sandstone, remarkably
uniform in its lithological character across the whole
State, generally accompanied by a few feet of shale
above and below, and in the borings at Kock Island,
Moline and Geneseo, in the west part of the State, it
has been found to contain a shaly stratum in its mid-
dle part.
143
THICKNESS AND POSITION OF THE ST. PETER SANDSTONE.
(Estimated from borings).
A. T.
Eock Island 145 to 200 feet. 465 610
Moline 200 " 367 567
Searles' Well (thickness unknown)... (?) " 315-(?)
Geneseo 220 " 428 648
Princeton '. 160 " 900 1060
LaSalle (shaly below at San Bede
College) (?)175 ;( 9071082
"Otica (exposed in bluff and eroded).. 40 " 522^82
Ottawa (partly exposed and eroded) . 130 " 483-345
Marseilles (?)200 " (?H?)
Peddicord's Well 275 " 350-75
Seneca (?)220 " (?)250-30
Hoge's Well 268 " 274-S
Morris' Driving Park Well (?) " 180-(?)
Minooka (?) " (?)82-(?)
Joliet 211 "' 91 302
The Magnesian Limestone. The Magnesian limestone is
the lowermost formation exposed in the State. It is an
impure, somewhat thin-bedded limestone, with many
seams of siliceous sand distributed through its entire
thickness, and sometimes with sand distributed through
the mass of the limestone. In the exposures between
Utica and LaSalle it often contains concretions of a
peculiar texture, which show siliceous sand grains imbed-
ded in a siliceous matrix, which evidently is a result of
infiltration. In many of the thin seams of clay, which
separate the limestone layers, there are marks of sun-
cracks, and in the mines of the Utica Cement Works there
are disclosed ripple marks of an unusual size measuring
by estimate 2 feet from crest to crest of the waves.
Westward from Joliet a bed of sand 100 feet in
thickness is developed in its upper part, and at Geneseo
the upper half of the formation has by the drillers been
reported as sandstone. Going from east to west the
144
formation increases in thickness, and at Rock Island
it is reported by Professor J. H. Southwell as having a
thickness of 811 feet.
Tlie Potsdam Series. The greater part of the Mag-
nesian limestone and all of the Potsdam series, being
known in the State only from borings, the proper
boundary between the two are of course somewhat
uncertain. If we make it at the point where the sandy,
light limestones are succeded by shales and sandstones,
with occasional less heavy deposits of limestone, the
formation, as far as explored, may be said to consist of
two sandstones separated by an intervening shale.
There is a noticeable correspondence in the strata
reported by the drillers from the deepest wells along the
east end of the section, and a strong flow of water has
been uniformly met with in a sandstone which occurs at
a depth of from 1700 to 2300 feet. Of course it is quite
possible that the correspondence in the strata is acci-
dental and that the "Potsdam sandstone" of the drillers
belongs to an underlying series which may be uncon-
formable to the Potsdam.
The Structural Features of the Section.
These are of the simplest kind and may be regarded
as typical of the structure found in the upper Mississippi
valley. We see two blocks of horizontal or only very
slightly inclined strata separated by a monoclinal fold.
The downthrow and the trough limb is on the west,
while the upthrow and the arch limb is on the east. The
total displacement of the Silurian strata amounts to
1,575 feet, while the carboniferous beds are only displaced
about 625 feet. The trend of the axis of disturbance is
considerably west of north, the strike of the outcrops of
the upturned coal measures being about N. 30 W. The
average dip in the displacement at LaSalle is about'
145
22 for the Silurian rocks and about 8 for the rocks
of the coal measures. The block of strata west of the
monocline is nearly horizontal in an east to west direc-
tion from Rock Island to Annawan and from Prince-
ton to LaSalle, but between Annawan and Princeton
there is a dip to the east of about 25 feet to the
mile, or there is a concealed displacement of that extent
between these two places. This dip may be partly ac-
counted for by the dip to the south, which is found
along the whole section. The block of strata on the
east of the monocline has a nearly uniform dip to