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Samuel Washington McCallie.

A preliminary report on the roads and road-building materials of Georgia online

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800 feet above the tide-water. It here shows a more advanced
stage of denudation than the part of the plain further south, and
is divided into many broad, elevated strips by shallow valleys.
These elevations have the appearance of low, very broad ridges,,
which seem to be the eroded portion of a once nearly horizontal
plateau. Near the coast, the surface becomes less deeply dissected
by running streams, which is evidently due to its more recent ele-



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TOPOGRAPHY OF GEORGIA



97



vation from the sea. The streams in their lower stretches trav-
erse low palmetto lands or swamps, which they flood during the
rainy season, making them both dangerous and difficult to cross.

An examination of a profile of any of the several railroads trav-
ersing the coastal plain shows, that the region is remarkably
level. This is exceptionally true near the coast, where the rail-
road frequently runs many miles in almost an air-line, "with a total
grade of only a few feet.

The topography of the Tertiary area or the Coastal Plain is very
favorable for highway-construction. The most serious difficulties
in the way of road-building are the numerous swamps and the low
depressions along the I'arger river valleys, where the roadway has
to be elevated, in order to insure proper drainage. Grading,
throughout the entire area, is reduced to a minimum, by reason
of the flatness of the country ; yet, there is usually sufficient varia-
tion in the surface to carry off all surface water.



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CHAPTER VIII



THE ROAD-BUILDING MATERIALS OF GEORGIA

The road-building materials of Georgia are quite abundant, and
pretty evenly distributed throughout the State. Nearly all the
varieties of stone used in highway-construction occur in large quan-
tities in many sections. It is questionable, whether any State in
the Union possesses a greater variety of road-building materials
than the State of Georgia. In describing these materials, the
State will be divided into three divisions, namely, the Paleozoic,
the Crystalline and the Tertiary areas, corresponding to the three
topographic divisions of the State, previously described.



ROAD MATERIALS OF THE PALEOZOIC AREA

The materials used for road-construction in this part of the
State, which comprises all or part of the ten counties, Polk, Floyd,
Bartow, Gordon, Murray, Whitfield, Catoosa, Chattooga, Walker
^nd Dade, consist of limestones, cherts, shales and sandstones.

Limestone, — The limestones of the area are very abundant, and
are well suited for macadamizing purposes. They are all of Silu-
rian age, and are divided geologically into three divisions, (i) The
Knox Dolomite, (2) The Chickamauga Limestone, and (3) The
Bangor or Mountain Limestone.

The Knox Dolomite is the most extensive of the three calca-
reous formations. It attains a thickness, in places, of more than
3,cxx) feet, and occurs in the form of a number of broad and nar-
row bands, traversing the area in a northesist-and-southwest direc-
tion, giving rise usually to broad, rounded ridges. The formation
consists largely of compact, heavy-bedded, light-gray magnesian
Jiraestone, often oolitic and always containing a considerable

(98)



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ROAD-BUILDING MATERIALS OF GEORGIA



99



amount of siliceous material, in the form of chert, hereafter to be
described. The silica present is in the form of flinty or cherty con-
cretions, unevenly distributed throughout the various beds. These
concretions sometimes occur in such abundance, as to form well-
defined layers, two feet or more in thickness. When the dolomite
weathers, the flinty or cherty material remains with the residual
clay, forming a mantle, often many feet in thickness. Where the
siliceous material is not abundant, the weathered product is usu-
ally a very tough, red, tenacious clay, which forms a very unsatis-
factory roadbed during wet weather. The Knox dolomite has
been used to a limited extent throughout the Paleozoic area for
road macadam ; but its most extensive use, up to the present, has
been for constructing retaining- walls, bridge-piers and culverts.
It has also been much used in places, notably Graysville and near
Cartersville, for making lime. The uniform texture and the semi-
crystalline structure of the dolomite well suit it for macadamizing
purposes. It would indeed be a difficult matter to find a calca-
reous deposit better adapted for road-material than some of the
beds of this formation. The stone is easily quarried, and is readily
crushed by the rock-breaker ; but it has, at the same time, suffi-
cient toughness, to form a durable wearing-surface. Its binding-
quality is all, that could be desired for a first-class road-material.

The Chickamauga Limestone overlies the Knox Dolomite. It
occurs in the form of narrow bands, more or less parallel, and
often valley-forming. The formation is so-called from Chicka-
mauga valley, where it reaches its greatest development. Its
various beds diflFer considerably, both in physical structure and
mineral composition. Dr. C. W. Hayes, of the U. S. Geological
Survey, in describing the formation says : — '

**It is, in its western exposure, a blue, flaggy, highly fossilifer-
ous limestone, with some local variations of minor importance.
From the eastern edge of Lookout mountain, there is a gradual
and uniform change toward the southeast. The change consists
in a decrease in the abundance of fossils and calcareous material



Bulletin No. 4. Geological Survey of Alabama, p. 42.



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ICXD ROAD-BUILDING MATERIALS OF GEORGIA

and an increase in argillaceous matter. The limestone becomes
more earthy and along the eastern edge of the area is red or dove-
colored and sparingly f ossiliferous. "

In the vicinity of Rockmart, the Chickamauga formation be-
comes highly argillaceous and forms a good quality of roofing
slate. Another peculiar feature of this formation is the heavy beds
of breccia formed of angular fragments of chert occurring in the
ridges south of Rockmart. This is pointed out by Dr. Spencer,
as showing a local unconformity at the close of the Knox Dolo-
mite epoch.' Much of the Chickamauga limestone weathers into
a shale, having a knotty structure. Such material is often used
for road-surfacing, without being crushed ; but its wear, on account
of its fragile nature, is usually unsatisfactory. The compact, blue
variety of this stone, on the other hand, makes an excellent mac-
adam. It has been extensively used for this purpose, both in
Chattanooga and in Chickamauga Park. The City Engineer of
Chattanooga, in speaking of this material, says: '*I regard this
stone superior to any material so far tried on our streets for mac-
adamizing purposes. It binds well, is comparatively free from
dust, and makes a hard, smooth road-surface." The Chickamauga
limestone varies from i,ooo to i,8oo feet in thickness, and is the
underlying rock in many of the narrow, fertile valleys of North-
west Georgia. In weathering, it forms a deep clay soil usually
difficult to drain ; and it makes a very unsatisfactory road-surface
in wet weather.

The Bangor or Mountain Limestone, which is of Carboniferous
Age, is a pure, dove-colored limestone, attaining a thickness of
about 900 feet. It is highly fossiliferous, and contains, in places^
crinoid stems in great abundance. The formation is well exposed
along the flanks of Pigeon and Lookout mountains, where it out-
crops beneath the sandstones. The extent of the area covered by
this formation is limited mainly to the narrow belts at the base of
the above named mountains ; and, as a consequence, it will prob-
ably never become of very great importance in road-construction •

The Paleozoic Group of Georgia, by J. W. Spencer ; 1893 ; p. 85.



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ROAD-BUILDING MATERIALS OF GEORGIA jqi

However, its use for macadam in the vicinity of Chattanooga
shows, that it is well suited for that purpose.

Chert, — The chert deposits of the Paleozoic area are quite ex-
tensive, and are widely distributed throughout the area. They oc-
cur in two different geological formations, namely, the Knox Dolo-
mite and the Fort Payne Chert, the latter formation being the
lowest member of the Carboniferous formation. The chert of the
Knox Dolomite is co-extensive with the dolomite itself, and is by
far the more important deposit of the two, both from a geological
and an economic standpoint. It occurs, as previously stated, in
the dolomite in the form of nodules ; and also in beds, frequently
several feet in thickness. In the weathering of the dolomite, the
chert remains as a residual product, in the form of gray flinty
nodules. This siliceous material frequently accumulates to the
depth of many feet, along the sides and slopes of ridges, where
it is often well exposed in railroad cuts. The chert of the Knox
Dolomite is an impure variety of flint, frequently containing more
or less calcareous material, and is readily crushed into sharp,
angular fragments. It has been extensively used for several years
for surfacing roads and streets, throughout North Georgia and
Tennessee. The material is well suited, for roads of light travel ;
but, where the traffic is heavy, it is inferior to limestone. It pos-
sesses an excellent binding-quality; but, after long drought and
much travel, it becomes somewhat dusty. In cities, this difficulty
can be readily overcome by sprinkling, which causes the loose par-
ticles to unite into a solid bond, forming a compact, smooth, dust-
less drive-way. The cheapness, with which this material can be
prepared, together with its admirable binding-qualities, makes it
one of the most important road-surfacing materials in the State.

The chert is often most favorably located for working and for
transportation. The railroads traversing the northwestern part of
the State have frequently exposed in their cuts, immense deposits
of this material, almost entirely free from clay and other foreign
materials. In such favored localities, it can be loaded on flat-cars
at a nominal cost and shipped to any part of the State. This ex-



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I02 ROAD-BUILDING MATERIALS OF GEORGIA

periment has been tried, in the last few years ; and, in a few in-
stances, the material has been shipped as far south as Macon, and
even to Savannah, where it was used for street-surfacing. With
reduced freight-rates, the cherts of the Paleozoic area of North
Georgia can be shipped from loo to acx) miles, and sold at a less
price than broken stone. The wide use of this material in the
future will no doubt depend, in a great measure, upon freight-rates.

The Fort Payne Chert is a siliceous limestone, varying in thick-
ness from 50 to 200 feet. Its lower layers consist largely of heavy
beds of chert, resembling very closely the chert of the Knox Dolo-
mite. This formation, like the Mountain Limestone, occurs along
the base of Lookout and Pigeon mountains. It also appears along
Taylor's ridge and Horn mountain, further east. The siliceous
nature of the formation is well exhibited along the Southern Rail-
way, about one mile south of Sugar Valley Station. The chert
here forms a heavy mantle, along the base of a ridge, which, in
places, must be many feet in thickness. Some distance up the
side of the slope is to be seen, in an excavation recently made for
road-material, the natural outcropping of heavy beds of chert.
The surfaces of the different layers are usually very rough, being
full of irregular cavities, which are evidently due to the weather-
ing pf the calcareous material formerly filling these spaces. The
chert of the Fort Payne formation is usually distinguished from
the Knox Dolomite cherts by its numerous fossils. It is, in places,
very fossiliferous, being made up largely of the stems of crinoids,
which are cemented together by a siliceous matrix. The siliceous
material of these two formations apparently possesses about equal
merits for road-surfacing. However, the wide extent and the
abundance of the Knox Dolomite makes it far more important as
a road-building material.

Shale, — The shales of the Paleozoic area belong to the Cam-
brian and Carboniferous formations. Those, deposited during the
Cambrian period, have been divided by Hayes into three divi-
sions,* (i) the Apison Shale, (2) the Rome Formation, and (3)

Geological Atlas, U. S. Gcol. Surv., Ringgold Folio (Ga.). P> a.



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ROAD-BUILDING MATERIALS OF GEORGIA



103:



the Connesauga Shale. These shales are generally valley-formings
and are commonly bounded on either side by Knox Dolomite-,
ridges. Chattooga, Peavine and Dogwood valleys are all under-
lain by Cambrian shales. The aggregate thickness of these shales-
is probably not less than 6,ocx) feet. The different divisions vary
considerably in color, mineral composition and physical structure.
The Apison shale is frequently sandy, and always banded with
brilliant colors of red, purple and yellow. The Rome formation*
is scarcely less brilliantly colored ; but it is far more siliceous,,
some of its beds consisting of an almost pure white quartzite,.
while the Connesauga shale is distinctly calcareous and, in places,
passes into an oolitic limestone.

The Floyd shale, which is of Carboniferous age, is well devel-^
oped in Floyd county immediately west of Rome. The formation
consists of calcareous shales with beds of sandstone and limestone.
The sandstone, when abundant, gives rise to low ridges as \xt
Texas and Dirt Town valleys.

The shales of the Paleozoic area are of but little economic im-
portance, as road-building materials. However, the shales in the
vicinity of Rome have been used to a considerable extent for road-
surfacing. This material makes a fair road surface ; but it is-
objectionable, on account of its rapid wear, and dusty condition
during the dry season, especially when there is much travel.

Siliceous Gravely which results from the weathering of the shales,,
often occurs in considerable abundance along the small streams
traversing the shale valleys. This material has also been used to-
a limited extent for road-surfacing in the vicinity of Rome. It
wears fairly well, and makes a more durable and dustless road-
covering than the shale itself.

The Limestones and Quartzites of the shales are suitable for road-
metal ; but their limited quantity practically prohibits them,
from ever becoming more than of local importance.

Sandstone. — The sandstones of the Paleozoic area are confined!
chiefly to Pigeon, Sand and Lookout mountains. They have been
divided into two divisions, namely, the Walden's Ridge sand*



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ROAD-BUILDING MATERIALS OF GEORGIA



stones and the Lookout Mountain sandstones. The former con-
stitute the surface-rock of the above named mountains ; while the
latter form the cliflF and escarpments along their slopes. The
two formations are sortiewhat similar in character. Each consists
of sandstone and conglomerate, interstratified with beds of shale.
The aggregate thickness of the formations is several hundred feet.
These standstones are easily crushed ; and consequently they are
of but little value for road-metal. The conglomerate^ associated
with the sandstone, forms beds of rounded gravel when weath-
ered, which is often fairly well suited for surfacing roadways.



THE ROAD-BUILDING MATERIALS OF THE CRYSTALLINE

BELT

The road-building materials of the Crystalline area consist of
granite, gneiss, diorite, schists, quartzite, marble, massive quartz
and trap rock. *

Granite. — The granite is very generally distributed throughout
the Crystalline area, where it occurs in the form of large intrusive
masses in the gneisses and schists. These granitic masses often
cover hundreds of acres, and occasionally, as in the case of Stone
Mountain, form dome-shaped masses, having an elevation of sev-
eral hundred feet above the surrounding country. Both the mus-
covite and the biotite varieties of granite occur in the Crystalline
area ; but the latter is much more abundant. In texture, these
granites differ widely. They vary from an exceedingly fine-
grained, homogeneous, monumental stone to a very coarse-grained
granite or pegmatite. The fine-grained varieties are quite exten-
sively quarried at several localities in the State for building and
monumental stone; and also for street-paving purposes. The
physical tests, which have been made on these granites, show that
they have great strength, and are therefore among the best of this
class of stone for road-material. However, it is not likely, that
they will ever ' become of general use for road-surfacing, where
there is such an abundance of other materials of superior quality.



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ROAD-BUILDING MATERIALS OF GEORGIA



105



Gneiss, — Gneiss is far more abundant in the Crystalline area
than granite; and, as a general rule, it is much more suitable for
Toad-material. The gneisses are divided mineralogically into two
well-known varieties, namely, the true gneiss made up of quartz,
feldspar and mica, and the hornblende-gneiss, which contains, in
addition to these minerals, hornblende as an essential constituent.
Horftblende-gneiss is generally superior to the true gneiss for road
purposes, on account of its finer texture and greater toughness. It
occurs, in places, throughout North Georgia, where it is found in
narrow belts underlying the so-called red lands. The great amount
of iron, which the rock carries, adds greatly to its binding quality.
This class of stone has been used to a limited extent on the streets
of Atlanta for macadamizing purposes. It wears well and is
usually free from dust. The true gneiss makes a fair road-sur-
facing material, when it is fine-grained and composed largely of
•quartz. Nevertheless, owing to the small amount of iron present,
its binding property is always inferior to that of the hornblende-
gneiss.

Diorite. — Diorite, which is more or less abundant throughout
North Georgia, is a green or dark-gray rock resembling very closely
in general appearance both the hornblende-gneiss and the horn-
blende-schist. It occurs mostly in the form of narrow belts or
zones, intercalated with the gneisses and schists. The essential
minerals of diorite are plagioclase and hornblende; but there are
.alm9st invariably other minerals present, such as pyrite and
magnetite. It is always holocrystalline and usually fine-grained.
The most of the diorites in Georgia, so far examined by the writer,
have a schistose or laminated structure, which has resulted from
pressure in the process of mountain making. This structure, which
is shown by microscopic examination to be due to the parallel
arrangement of the individual minerals constituting the rock, has
a tendency to weaken it along certain lines and thereby injure it
for road-macadamizing purposes. The diorites, when fine-grained
and not too distinctly laminated, make an admirable road-material,
second only to diabase. The toughness, hardness and binding



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106 ROAD'BUILDING MATERIALS OF GEORGIA

quality of this stone are all excellent. A large exposure of this
rock is to be seen on the Southern Railway, a short distance west
of Dallas ; and also at many other places, in the Crystalline area^
which will be described in full in speaking of the varieties of stone,,
suitable for road-material, in the several counties of the State.

Trap or Diabase. — Trap rock is very generally distributed
throughout the Crystalline area. It occurs always in the form of
dikes, which vary in thickness from a few inches to several rods.
These dikes, which have originated from the filling up of fissures
by molten matter forced up from below, have a generally north-
west-and-southeast trend and a nearly vertical dip. They almost
invariably cut the gneisses and the schists at a considerable angle^
and rarely ever show any evidence of shearing, or any crust
movement since their formation. Geologically speaking, the
dikes are all of recent origin. They probably date from the Jura-
trias period, and are presumably contemporaneous in origin with
the Palisades of the Hudson and the trap dikes of the Connecticut
valley. It seems quite likely, that many of these larger dikes
furnished surface lava-flows in many places of North Georgia dur-
ing the Jura-trias period. However, as far as the writer knows at
present, there exists nowhere within the Crystalline area any rem-
nant of such surface overflows, by which this statement can be
verified. This negative evidence, however, cannot be taken as
conclusive; as such flows might have actually existed during the
Jura-trias period, and have since been entirely removed by denu-
dation. Prof. I. C. Russell, in speaking of the trap dikes, says : ^
*'In the greater portion of the area along the Atlantic coast, that
was fractured, so as to admit of the upward passage of molten
rocks from beneath, extensive and deep erosion has occurred, and
only truncated dikes and remnants of igneous sheets remain."

All the larger dikes of Georgia, so far examined, are usually
quite uniform in thickness, and frequently extend for many miles
with but few interruptions. A good example of the large dikes
is to be seen in a cut on the Central Railway, a few miles east of

* Volcanoes of North America, p. 44.



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ROAD-BUILDING MATERIALS OF GEORGIA



107



Newnan. This dike continues for about 65 miles in a southeast-
erly direction, through Coweta, Meriwether and Talbot counties,
finally disappearing beneath the Columbia sands, about four miles
south of Talbotton. In this distance, there occur a number of
breaks or interruptions, a mile or more in length, which are due
either to an actual discontinuity of the dike, or its burial beneath
the residual decay from the including gneisses and schists. Par-
rallel with the main dikes, are usually found one or more small
dikes, which may vary from an inch to several feet in thickness.
The rock forming the smaller dikes is always fine-grained, re-
sembling very closely the contact edges of the larger dikes. When
the dikes become of large size, as in the case of the one traversing
Jones and Jasper counties, they frequently form low, well-rounded
ridges, whose surfaces are covered with innumerable rounded
bowlders, varying from a few inches to many feet in diameter.
In some instances, the large dikes form cataracts or falls in small
streams ; but this is not of common occurrence. The best expos-
ures of dikes is to be seen along the several railroads traversing
the Crystalline area. Here, they are frequently exposed in cuts,
to the depth of twenty feet, or more, and their relation to the
gneisses and schists, together with their mode of weathering, can
be easily studied. In such artificial excavations, they are fre-
quently more or less numerous. At one point on the Georgia
Railroad, near Covington, there are to be seen, in a distance of
less than two miles, as many as seven dikes in the various cuts.
The smallof of these are so completely disintegrated to the depth
of several feet, that their presence is not indicated on the surface,
in the cultivated fields.

The rocks forming the trap dikes of Georgia are all typical dia-
base, consisting of plagioclase and augite, with a number of ac-
cessory minerals, the most common being olivine and magnetite.
These rocks are of dark-gray or black color, usually fine-grained
and quite diflScult to break with a hammer. As a road-surfacing
material, this class of rocks has no equal. Its great hardness and



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I08 ROAD-BUILDING MATERIALS OF GEORGIA

its remarkable toughness, together with its excellent binding
quality, make it an ideal road-building material.

There are a number of localities along the several railroads of
North Georgia, where quarries of this superior road-material can
be opened up at a small cost, and the stone can be readily shipped
to all parts of the State, for both street and road purposes. The
trap rock of the State is almost unknown ; and, as a consequence,
it has had no use in road-construction.

Hornblende-SchisL — Associated with the diorites, and frequently
indistinguishable from them, except by microscopic examination,


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Online LibrarySamuel Washington McCallieA preliminary report on the roads and road-building materials of Georgia → online text (page 8 of 22)