CHAPTER V.

THE GLACIAL PERIOD.

The eastern part of the area with which this report deals, is covered

with a mantle of drift which, as already pointed out, has greatly

modified the details of its topography. To the consideration of the

drift and its history attention is now turned.

_The drift._--The drift consists of a body of clay, sand, gravel and

bowlders, spread out as a cover of unequal thickness over the rock

formations beneath. These various classes of material may be confusedly

commingled, or they may be more or less distinctly separated from one

another. When commingled, all may be in approximately equal proportions,

or any one may predominate over any or all the others to any extent.

It was long since recognized that the materials of the drift did not

originate where they now lie, and that, in consequence, they sustain no

genetic relationship to the strata on which they rest. Long before the

drift received any special attention from geologists, it was well known

that it had been transported from some other locality to that where it

now occurs. The early conception was that it had been drifted into its

present position from some outside source by water. It was this

conception of its origin which gave it the name of _drift_. It is now

known that the drift was deposited by glacier ice and the waters which

arose from its melting, but the old name is still retained.

Clearly to understand the origin of the drift, and the method by which

it attained its present distribution, it may be well to consider some

elementary facts and principles concerning climate and its effects, even

at the risk of repeating what is already familiar.

_Snow fields and ice sheets._--The temperature and the snowfall of a

region may stand in such a relation to each other that the summer's heat

may barely suffice to melt the winter's snow. If under these

circumstances the annual temperature were to be reduced, or the fall of

snow increased, the summer's heat would fail to melt all the winter's

snow, and some portion of it would endure through the summer, and

through successive summers, constituting a perennial snow-field. Were

this process once inaugurated, the depth of the snow would increase from

year to year. The area of the snow-field would be extended at the same

time, since the snow-field would so far reduce the surrounding

temperature as to increase the proportion of the annual precipitation

which fell as snow. In the course of time, and under favorable

conditions, the area of the snow-field would attain great dimensions,

and the depth of the snow would become very great.

As in the case of existing snow fields the lower part of the snow mass

would eventually be converted into ice. Several factors would conspire

to this end. 1. The pressure of the overlying snow would tend to

compress the lower portion, and snow rendered sufficiently compact by

compression would be regarded as ice. 2. Water arising from the melting

of the surface snow by the sun's heat, would percolate through the

superficial layers of snow, and, freezing below, take the form of ice.

3. On standing, even without pressure or partial melting, snow appears

to undergo changes of crystallization which render it more compact. In

these and perhaps other ways, a snow-field becomes an ice-field, the

snow being restricted to its surface.

Eventually the increase in the depth of the snow and ice in a snow-field

will give rise to new phenomena. Let a snow and ice field be assumed in

which the depth of snow and ice is greatest at the center, with

diminution toward its edges. The field of snow, if resting on a level

base, would have some such cross-section as that represented in the

diagram, Fig. 27.

When the thickness of the ice has become considerable, it is evident

that the pressure upon its lower and marginal parts will be great. We

are wont to think of ice as a brittle solid. If in its place there were

some plastic substance which would yield to pressure, the weight of the

ice would cause the marginal parts to extend themselves in all

directions by a sort of flowing motion.

[Illustration: Fig. 27.--Diagrammatic cross-section of a field of ice

and snow (c) resting on a level base A-B.]

Under great pressure, many substances which otherwise appear to be

solid, exhibit the characteristics of plastic bodies. Among the

substances exhibiting this property, ice is perhaps best known. Brittle

and resistant as it seems, it may yet be molded into almost any

desirable form if subjected to sufficient pressure, steadily applied

through long intervals of time. The changes of form thus produced in ice

are brought about without visible fracture. Concerning the exact nature

of the movement, physicists are not agreed; but the result appears to be

essentially such as would be brought about if the ice were capable of

flowing, with extreme slowness, under great pressure continuously

applied.

In the assumed ice-field, there are the conditions for great pressure

and for its continuous application. If the ice be capable of moving as a

plastic body, the weight of the ice would induce gradual movement

outward from the center of the field, so that the area surrounding the

region where the snow accumulated would gradually be encroached upon by

the spreading of the ice. Observation shows that this is what takes

place in every snow-field of sufficient depth. Motion thus brought about

is glacier motion, and ice thus moving is glacier ice.

Once in motion, two factors would determine the limit to which the ice

would extend itself: (1) the rate at which it advances; and (2) the rate

at which the advancing edge is wasted. The rate of advance would depend

upon several conditions, one of which, in all cases, would be the

pressure of the ice which started and which perpetuates the motion. If

the pressure be increased the ice will advance more rapidly, and if it

advance more rapidly, it will advance farther before it is melted. Other

things remaining constant, therefore, increase of pressure will cause

the ice-sheet to extend itself farther from the center of motion.

Increase of snowfall will increase the pressure of the snow and ice

field by increasing its mass. If, therefore, the precipitation over a

given snow-field be increased for a period of years, the ice-sheet's

marginal motion will be accelerated, and its area enlarged. A decrease

of precipitation, taken in connection with unchanged wastage would

decrease the pressure of the ice and retard its movement. If, while the

rate of advance diminished, the rate of wastage remained constant, the

edge of the ice would recede, and the snow and ice field be contracted.

The rate at which the edge of the advancing ice is wasted depends

largely on the climate. If, while the rate of advance remains constant,

the climate becomes warmer, melting will be more rapid, and the ratio

between melting and advance will be increased. The edge of the ice will

therefore recede. The same result will follow, if, while temperature

remains constant, the atmosphere becomes drier, since this will increase

wastage by evaporation. Were the climate to become warmer and drier at

the same time, the rate of recession of the ice would be greater than if

but one of these changes occurred.

If, on the other hand, the temperature over and about the ice field be

lowered, melting will be diminished, and if the rate of movement be

constant, the edge of the ice will advance farther than under the

earlier conditions of temperature, since it has more time to advance

before it is melted. An increase in the humidity of the atmosphere,

while the temperature remains constant, will produce the same result,

since increased humidity of the atmosphere diminishes evaporation. A

decrease of temperature, decreasing the melting, and an increase of

humidity, decreasing the evaporation, would cause the ice to advance

farther than either change alone, since both changes decrease the

wastage. If, at the same time that conditions so change as to increase

the rate of movement of the ice, climatic conditions so change as to

reduce the rate of waste, the advance of the ice before it is melted

will be greater than where only one set of conditions is altered. If,

instead of favoring advance, the two series of conditions conspire to

cause the ice to recede, the recession will likewise be greater than

when but one set of conditions is favorable thereto.

Greenland affords an example of the conditions here described. A large

part of the half million or more square miles which this body of land is

estimated to contain, is covered by a vast sheet of snow and ice,

thousands of feet in thickness. In this field of snow and ice, there is

continuous though slow movement. The ice creeps slowly toward the

borders of the island, advancing until it reaches a position where the

climate is such as to waste (melt and evaporate) it as rapidly as it

advances.

The edge of the ice does not remain fixed in position. There is reason

to believe that it alternately advances and retreats as the ratio

between movement and waste increases or decreases. These oscillations in

position are doubtless connected with climatic changes. When the ice

edge retreats, it may be because the waste is increased, or because the

snowfall is decreased, or both. In any case, when the ice edge recedes

from the coast, it tends to recede until its edge reaches a position

where the melting is less rapid than in its former position, and where

the advance is counterbalanced by the waste. This represents a condition

of equilibrium so far as the edge of the ice is concerned, and here the

edge of the ice would remain so long as the conditions were unchanged.

When for a period of years the rate of melting of the ice is diminished,

or the snowfall increased, or both, the ice edge advances to a new line

where melting is more rapid than at its former edge. The edge of the ice

would tend to reach a position where waste and advance balance. Here its

advance would cease, and here its edge would remain so long as climatic

conditions were unchanged.

If the conditions determining melting and flowage be continually

changing, the ice edge will not find a position of equilibrium, but will

advance when the conditions are favorable for advance, and retreat when

the conditions are reversed.

Not only the edge of the ice in Greenland, but the ends of existing

mountain glaciers as well, are subject to fluctuation, and are delicate

indices of variations in the climate of the regions where they occur.

_The North American ice sheet._--In an area north of the eastern part of

the United States and in another west of Hudson Bay it is believed that

ice sheets similar to that which now covers Greenland began to

accumulate at the beginning of the glacial period. From these areas as

centers, the ice spread in all directions, partly as the result of

accumulation, and partly as the result of movement induced by the weight

of the ice itself.

The ice sheets spreading from these centers came together south of

Hudson's bay, and invaded the territory of the United States as a single

sheet, which, at the time of its greatest development, covered a large

part of our country (Plate XXXIII), its area being known by the extent

of the drift which it left behind when it was melted. In the east, it

buried the whole of New England, most of New York, and the northern

parts of New Jersey and Pennsylvania. Farther west, the southern margin

of the ice crossed the Ohio river in the vicinity of Cincinnati, and

pushed out over the uplands a few miles south of the river. In Indiana,

except at the extreme east, its margin fell considerably short of the

Ohio; in Illinois it reached well toward that river, attaining here its

most southerly latitude. West of the Mississippi, the line which marks

the limit of its advance curves to the northward, and follows, in a

general way, the course of the Missouri river. The total area of the

North American ice sheet, at the time of its maximum development, has

been estimated to have been about 4,000,000 square miles, or about ten

times the estimated area of the present ice-field of Greenland.

Within the general area covered by the ice, there is an area of several

thousand square miles, mainly in southwestern Wisconsin, where there is

no drift. The ice, for some reason, failed to cover this _driftless

area_ though it overwhelmed the territory on all sides.

[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,

PL. XXXIII.

The North American Ice Sheet, at the time of maximum development.]

[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,

PL. XXXIV.

View from the north of the Owl's Head, a hill two miles north of east of

Merrimac, which has been shaped by the ice. The side to the left is the

stone side.]

Plate II shows the limit of ice advance in the area here described. The

region may have been affected by the ice of more than one glacial epoch,

but the chief results now observable were effected during the last, and

the others need not be considered.

_The Work of Glacier Ice._

As the edge of an ice sheet, or as the end of a glacier, retreats, the

land which it has previously covered is laid bare, and the effects which

the passage of the ice produced may be seen. In some cases one may

actually go back a short distance beneath the ice now in motion, and see

its mode of work and the results it is effecting. The beds of living

glaciers, and the beds which glaciers have recently abandoned, are found

to present identical features. Because of their greater accessibility,

the latter offer the better facilities for determining the effects of

glaciation.

The conspicuous phenomena of abandoned glacier beds fall into two

classes, (1) those which pertain to the bed rock over which the ice

moved, and (2) those which pertain to the drift left by the ice.

_Erosive work of the ice._--_Effect on topography._--The leading

features of the rock bed over which glacier ice has moved, are easily

recognized. Its surface is generally smoothed and polished, and

frequently marked by lines (striae) or grooves, parallel to one another.

An examination of the bottom of an active glacier discloses the method

by which the polishing and scoring are accomplished.

The lower surface of the ice is thickly set with a quantity of clay,

sand, and stony material of various grades of coarseness. These earthy

and stony materials in the base of the ice are the tools with which it

works. Thus armed, the glacier ice moves slowly forward, resting down

upon the surfaces over which it passes with the whole weight of its

mass, and the grinding action between the stony layer at the base of the

ice and the rock bed over which it moves, is effective. If the material

in the bottom of the ice be fine, like clay, the rock bed is polished.

If coarser materials, harder than the bed-rock, be mingled with the

fine, the rock bed of the glacier will be scratched as well as polished.

If there are bowlders in the bottom of the ice they may cut grooves or

gorges in the underlying rock. The grooves may subsequently be polished

by the passage over and through them of ice carrying clay or other fine,

earthy matter.

All these phases of rock wear may be seen about the termini of receding

glaciers, on territory which they have but recently abandoned. There can

thus be no possible doubt as to the origin of the polishing, planing and

scoring.

There are other peculiarities, less easily defined, which characterize

the surface of glacier beds. The wear effected is not confined to the

mere marking of the surface over which it passes. If prominences of rock

exist in its path, as is often the case, they oppose the movement of the

ice, and receive a corresponding measure of abrasion from it. If they be

sufficiently resistant they may force the ice to yield by passing over

or around them; but if they be weak, they are likely to be destroyed.

As the ice of the North American ice sheet advanced, seemingly more

rigid when it encountered yielding bodies, and more yielding when it

encountered resistant ones, it denuded the surface of its loose and

movable materials, and carried them forward. This accumulation of earthy

and stony debris in the bottom of the ice, gave it a rough and grinding

lower surface, which enabled it to abrade the land over which it passed

much more effectively than ice alone could have done. Every hill and

every mound which the ice encountered contested its advance. Every

sufficiently resistant elevation compelled the ice to pass around or

over it; but even in these cases the ice left its marks upon the surface

to which it yielded. The powerful pressure of pure ice, which is

relatively soft, upon firm hills of rock, which are relatively hard,

would effect little. The hills would wear the ice, but the effect of the

ice on the hills would be slight. But where the ice is supplied with

earthy and stony material derived from the rock itself, the case is

different. Under these conditions, the ice, yielding only under great

pressure and as little as may be, rubs its rock-shod base over every

opposing surface, and with greatest severity where it meets with

greatest resistance. Its action may be compared to that of a huge

"flexible-rasp" fitting down snugly over hills and valleys alike, and

working under enormous pressure.

The abrasion effected by a moving body of ice under such conditions

would be great. Every inch of ice advance would be likely to be attended

by loss to the surface of any obstacle over or around which it is

compelled to move. The sharp summits of the hills, and all the angular

rugosities of their surfaces would be filed off, and the hills smoothed

down to such forms as will offer progressively less and less resistance.

If the process of abrasion be continued long enough, the forms, even of

the large hills, may be greatly altered, and their dimensions greatly

reduced. Among the results of ice wear, therefore, will be a lowering of

the hills, and a smoothing and softening of their contours, while their

surfaces will bear the marks of the tools which fashioned them, and will

be polished, striated or grooved, according to the nature of the

material which the ice pressed down upon them during its passage. Figs.

28 and 29 show the topographic effects which ice is likely to produce by

erosion. Plate XXXIV is a hill two miles northeast of Merrimac, which

shows how perfectly the wear actually performed corresponds to that

which might be inferred.

[Illustration: Fig. 28.--A hill before the ice passes over it.]

A rock hill was sometimes left without covering of drift after having

been severely worn by the ice. Such a hill is known as a _roche

moutonnee_. An example of this type of hill occurs three miles north of

east of Baraboo at the point marked z on Plate XXXVII. This hill,

composed of quartzite, is less symmetrical than those shown in Figs. 28

and 29. Its whole surface, not its stoss side only, has been smoothed

and polished by the ice. This hill is the most accessible, the most

easily designated, and, on the whole, the best example of a _roche

moutonnee_ in the region, though many other hills show something of the

same form.

[Illustration: Fig. 29.--The same hill after it has been eroded by the

ice. A the stoss side. B the lee side.]

It was not the hills alone which the moving ice affected. Where it

encountered valleys in its course they likewise suffered modification.

Where the course of a valley was parallel to the direction of the ice

movement, the ice moved through it. The depth of moving ice is one of

the determinants of its velocity, and because of the greater depth of

ice in valleys, its motion here was more rapid than on the uplands

above, and its abrading action more powerful. Under these conditions the

valleys were deepened and widened.

Where the courses of the valleys were transverse to the direction of ice

movement, the case was different. The ice was too viscous to span the

valleys, and therefore filled them. In this case it is evident that the

greater depth of the ice in the valley will not accelerate its motion,

since the ice in the valley-trough and that above it are in a measure

opposed. If left to itself, the ice in the valley would tend to flow in

the direction of the axis of the valley. But in the case under

consideration, the ice which lies above the valley depression is in

motion at right angles to the axis of the valley. Under these

circumstances three cases might arise:

(1) If the movement of the ice sheet over the valley were able to push

the valley ice up the farther slope, and out on the opposite highland,

this work would retard the movement of the upper ice, since the

resistance to movement would be great. In this case, the thickness of

the ice is not directly and simply a determinant of its velocity. Under

these conditions the bottom of the valley would not suffer great

erosion, since ice did not move along it; but that slope of the valley

against which the ice movement was projected would suffer great wear

(Fig. 30). The valley would therefore be widened, and the slope

suffering greatest wear would be reduced to a lower angle. Shallow

valleys, and those possessing gentle slopes, favor this phase of ice

movement and valley wear.

[Illustration: Fig. 30.--Diagram showing effect on valley of ice moving

transversely across it.]

(2) The ice in the valley might become stationary, in which case it

might serve as a bridge for the upper ice to cross on (Fig. 31). In this

case also the total thickness of ice will not be a determinant of its

velocity, for it is the thickness of the moving ice only, which

influences the velocity. In this case the valley would not suffer much

wear, so long as this condition of things continued. Valleys which have

great depth relative to the thickness of the ice, and valleys whose

slopes are steep, favor this phase of movement.

(3) In valleys whose courses are transverse to the direction of ice

movement, transverse currents of ice may exist, following the direction

of the valleys. If the thickness of the ice be much greater than the

depth of the valley, if the valley be capacious, and if one end of it be

open and much lower than the other, the ice filling it may move along

its axis, while the upper ice continues in its original course at right

angles to the valley. In this case the valley would be deepened and

widened, but this effect would be due to the movement along its course,

rather than to that transverse to it.

[Illustration: Fig. 31--Diagram to illustrate case where ice fills a

valley (C) and the upper ice then moves on over the filling.]

If the course of a valley were oblique to the direction of ice movement,

its effect on the movement of ice would be intermediate between that of

valleys parallel to the direction of movement, and those at right angles

to it.

It follows from the foregoing that the corrasive effects of ice upon the

surface over which it passed, were locally dependent on pre-existent

topography, and its relation to the direction of ice movement. In

general, the effort was to cut down prominences, thus tending to level

the surface. But when it encountered valleys parallel to its movement

they were deepened, thus locally increasing relief. Whether the

reduction of the hills exceeded the deepening of the valleys, or whether

the reverse was true, so far as corrasion alone is concerned, is

uncertain. But whatever the effect of the erosive effect of ice action

upon the total amount of relief, the effect upon the contours was to

make them more gentle. Not only were the sharp hills rounded off, but

even the valleys which were deepened were widened as well, and in the

process their slopes became more gentle. A river-erosion topography,

modified by the wearing (not the depositing) action of the ice, would be

notably different from the original, by reason of its gentler slopes and

softer contours (Figs. 28 and 29).

_Deposition by the ice. Effect on topography._--On melting, glacier ice

leaves its bed covered with the debris which it gathered during its

movement. Had this debris been equally distributed on and in and beneath

the ice during its movement, and had the conditions of deposition been

everywhere the same, the drift would constitute a mantle of uniform

thickness over the underlying rock. Such a mantle of drift would not

greatly alter the topography; it would simply raise the surface by an

amount equal to the thickness of the drift, leaving elevations and

depressions of the same magnitude as before, and sustaining the same

relations to one another. But the drift carried by the ice, in whatever

position, was not equally distributed during transportation, and the

conditions under which it was deposited were not uniform, so that it

produced more or less notable changes in the topography of the surface

on which it was deposited.

The unequal distribution of the drift is readily understood. The larger

part of the drift transported by the ice was carried in its basal

portion; but since the surface over which the ice passed was variable,

it yielded a variable amount of debris to the ice. Where it was hilly,

the friction between it and the ice was greater than where it was plain,

and the ice carried away more load. From areas where the surface was

overspread by a great depth of loose material favorably disposed for

removal, more debris was taken than from areas where material in a

condition to be readily transported was meager. Because of the

topographic diversity and lithological heterogeneity of the surface of

the country over which it passed, some portions of the ice carried much

more drift than others, and when the ice finally melted, greater depths

of drift were left in some places than in others. Not all of the

material transported by the ice was carried forward until the ice

melted. Some of it was probably carried but a short distance from its

original position before it lodged. Drift was thus accumulating at some

points beneath the ice during its onward motion. At such points the

surface was being built up; at other points, abrasion was taking place,

and the surface was being cut down. The drift mantle of any region does

not, therefore, represent simply the material which was on and in and

beneath the ice of that place at the time of its melting, but it

represents, in addition, all that lodged beneath the ice during its

movement.

The constant tendency was for the ice to carry a considerable part of

its load forward toward its thinned edge, and there to leave it. It

follows that if the edge of the ice remained constant in position for

any considerable period of time, large quantities of drift would have

accumulated under its marginal portion, giving rise to a belt of

relatively thick drift. Other things being equal, the longer the time

during which the position of the edge was stationary, the greater the

accumulation of drift. Certain ridge-like belts where the drift is

thicker than on either hand, are confidently believed to mark the

position where the edge of the ice-sheet stood for considerable periods

of time.

Because of the unequal amounts of material carried by different parts of

the ice, and because of the unequal and inconstant conditions of

deposition under the body of the ice and its edge, the mantle of drift

has a very variable thickness; and a mantle of drift of variable

thickness cannot fail to modify the topography of the region it covers.

The extent of the modification will depend on the extent of the

variation. This amounts in the aggregate, to hundreds of feet. The

continental ice sheet, therefore, modified the topography of the region

it covered, not only by the wear it effected, but also by the deposits

it made.

In some places it chanced that the greater thicknesses of drift were

left in the positions formerly marked by valleys. Locally the body of

drift was so great that valleys were completely filled, and therefore

completely obliterated as surface features. Less frequently, drift not

only filled the valleys but rose even higher over their former positions

than on either side. In other places the greater depths of drift,

instead of being deposited in the valleys, were left on pre-glacial

elevations, building them up to still greater heights. In short, the

mantle of drift of unequal thickness was laid down upon the rock

surface in such a manner that the thicker parts sometimes rest on hills

and ridges, sometimes on slopes, sometimes on plains, and sometimes in

valleys.

[Illustration: Fig. 32.--Diagrammatic section showing relation of drift

to underlying rock, where the drift is thick relative to the relief of

the rock. a and b represent the location of post-glacial valleys.]

These relations are suggested by Figs. 32 and 33. From them it will be

seen that in regions where the thickness of the drift is great, relative

to the relief of the underlying rock, the topography may be completely

changed. Not only may some of the valleys be obliterated by being

filled, but some of the hills may be obliterated by having the lower

land between them built up to their level. In regions where the

thickness of the drift is slight, relative to the relief of the rock

beneath, the hills cannot be buried, and the valleys cannot be

completely filled, so that the relative positions of the principal

topographic features will remain much the same after the deposition of

the drift, as before (Fig. 33).

[Illustration: Fig. 33.--Diagrammatic section showing relation of drift

to underlying rock where the drift is thin relative to the relief of the

underlying rock.]

In case the pre-glacial valleys were filled and the hills buried, the

new valleys which the surface waters will in time cut in the drift

surface will have but little correspondence in position with those

which existed before the ice incursion. A new system of valleys, and

therefore a new system of ridges and hills, will be developed, in some

measure independent of the old. These relations are illustrated by Fig.

32.

Inequalities in the thickness of drift lead to a still further

modification of the surface. It frequently happened that in a plane or

nearly plane region a slight thickness of drift was deposited at one

point, while all about it much greater thicknesses were left. The area

of thin drift would then constitute a depression, surrounded by a higher

surface built up by the thicker deposits. Such depressions would at

first have no outlets, and are therefore unlike the depressions shaped

by rain and river erosion. The presence of depressions without outlets

is one of the marks of a drift-covered (glaciated) country. In these

depressions water may collect, forming lakes or ponds, or in some cases

only marshes and bogs.

DIRECTION OF ICE MOVEMENT.

The direction in which glacier ice moved may be determined in various

ways, even after the ice has disappeared. The shapes of the rock hills

over which the ice passed, the direction from which the materials of the

drift came, and the course of the margin of the drift, all show that the

ice of south central Wisconsin was moving in a general southwest

direction. In the rock hills, this is shown by the greater wear of their

northeast ("stoss") sides (Plate XXXIV). From the course of the drift

margin, the general direction of movement may be inferred when it is

remembered that the tendency of glacier ice on a plane surface is to

move at right angles to its margin.

For the exact determination of the direction of ice movement, recourse

must be had to the striae on the bed-rock. Were the striated rock surface

perfectly plane, and were the striae even lines, they would only tell

that the ice was moving in one of two directions. But the rock surface

is not usually perfectly plane, nor the striae even lines, and between

the two directions which lines alone might suggest, it is usually

possible to decide. The minor prominences and depressions in the rock

surface were shaped according to the same principles that govern the

shaping of hills (Fig. 29) and valleys (Fig. 30); that is, the stoss

sides of the minor prominences, and the distal sides of small

depressions suffered the more wear. With a good compass, the direction

of the striae may be measured to within a fraction of a degree, and thus

the direction of ice movement in a particular place be definitely

determined. The striae which have been determined about Baraboo are shown

on Plate II.

_Effect of topography on movement._--The effect of glaciation on

topography has been sketched, but the topography in turn exerted an

important influence on the direction of ice movement. The extreme degree

of topographic influence is seen in mountain regions like the Alps,

where most of the glaciers are confined strictly to the valleys.

As an ice sheet invades a region, it advances first and farthest along

the lines of least resistance. In a rough country with great relief,

tongues or lobes of ice would push forward in the valleys, while the

hills or other prominences would tend to hold back or divide the onward

moving mass. The edge of an ice sheet in such a region would be

irregular. The marginal lobes of ice occupying the valleys would be

separated by re-entrant angles marking the sites of hills and ridges.

If the ice crossed a plane surface above which rose a notable ridge or

hill, the first effect of the hill would be to indent the ice. The ice

would move forward on either side, and if its thickness became

sufficiently great, the parts moving forward on either side would again

unite beyond it. A hill thus surrounded by ice is a nunatak. Later, as

the advancing mass of ice became thicker, it might completely cover the

hill; but the thickness of ice passing over the hill would be less than

that passing on either side by an amount equal to the height of the

hill. It follows that as ice encounters an isolated elevation, three

stages in its contest with the obstruction may be recognized: (1) the

stage when the ridge or hill acts as a wedge, dividing the moving ice

into lobes, Fig. 34; (2) the nunatak stage, when the ice has pushed

forward and reunited beyond the hill, Fig. 35; (3) the stage when the ice

has become sufficiently deep to cover the hill.

[Illustration: Fig. 34.--Diagrammatic representation of the effect of a

hill on the edge of the ice.]

After the ice has disappeared, the influence of the obstruction might be

found in the disposition of the drift. If recession began during the

first stage, that is, when the ice edge was separated into lobes, the

margin of the drift should be lobate, and would loop back around the

ridge from its advanced position on either side. If recession began

during the second stage, that is, when the lobes had become confluent

and completely surrounded the hill, a _driftless area_ would appear in

the midst of drift. If recession began during the third stage, that is,

after the ice had moved on over the obstruction, the evidence of the

sequence might be obliterated; but if the ice moved but a short distance

beyond the hill, the thinner ice over the hill would have advanced less

far than the thicker ice on either side (Fig. 35), and the margin of

the drift would show a re-entrant pointing back toward the hill, though

not reaching it. All these conditions are illustrated in the Devil's

lake region.

[Illustration: Fig. 35.--Same as Fig. 34, when the ice has advanced

farther.]

_Limit of the Ice._

The region under description is partly covered with drift, and partly

free from it. The limit of the ice, at the time of its maximum expansion

is well defined at many points, and the nature and position of the drift

limit are so unique as to merit attention (see Plates II and XXXVII).

They illustrate many of the principles already discussed.

The ice which covered the region was the western margin of the Green Bay

lobe (Fig. 36) of the last continental ice sheet. Its limit in this

region is marked by a ridge-like accumulation of drift, the _terminal

moraine_, which here has a general north-south direction. The region

may have been affected by the ice of more than one epoch, but since the

ice of the last epoch advanced as far to the west in this region as that

of any earlier epoch, the moraine is on the border between the glaciated

country to the east, and the driftless area to the west (Plates I and

II). That part of the moraine which lies west of the Wisconsin river

follows a somewhat sinuous course from Kilbourn City to a point a short

distance north of Prairie du Sac. The departures from this general

course are especially significant of the behavior of glacier ice.

[Illustration: Fig. 36.--Map showing relations of lobes of ice during

the Wisconsin ice epoch, to the driftless area.]

In the great depression between the quartzite ranges, the moraine bends

westward, showing that the ice advanced farther on the lowlands than on

the ridges. As the moraine of this low area approaches the south range,

it curves to the east. At the point southwest of Baraboo where the

easterly curve begins to show itself, the moraine lies at the north base

of the quartzite range; but as it is traced eastward, it is found to lie

higher and higher on the slope of the range, until it reaches the crest

nearly seven miles from the point where the eastward course was assumed.

At this point it crosses the range, and, once across the crest, it turns

promptly to the westward on the lower land to the south. Here the ice

advanced up the valley between the East bluff (east of the lake) and the

Devil's nose (Plate XXXVII), again illustrating the fact that lowlands

favor ice advance. The valley between the Devil's nose and the East

bluff is a narrow one, and the ice advanced through it nearly to the

present site of the lake. Meanwhile the restraining influence of the

"nose" was making itself felt, and the margin of the ice curved back

from the bottom of the bluff near Kirkland, to the top of the bluff at

the end of the nose. Here the edge of the ice crossed the point of the

nose, and after rounding it, turned abruptly to the west. Thence its

edge lay along the south slope of the ridge, descending from the crest

of the ridge at the nose, to the base of the ridge two miles farther

west. Here the ice reached its limit on the lowland, and its edge, as

marked by the moraine, turned southward, reaching the Wisconsin river

about a mile and a half above Prairie du Sac.

The course of the terminal moraine across the ridges is such as the

margin of the ice would normally have when it advanced into a region of

great relief. The great loop in the moraine with its eastern extremity

at k, Plate XXXVII, is explained by the presence of the quartzite

ridge which retarded the advancing ice while it moved forward on either

side. The minor loop around the Devil's nose is explained in the same

way. Both the main loop, and the smaller one on the nose, illustrate

the point made earlier in the text.

The narrow and curious loop at m, is of a slightly different origin,

though in principle the same. It is in the lee of a high point in the

quartzite ridge. The ice surmounted this point, and descended its

western slope; but the thickness of the ice passing over the summit was

so slight that it advanced but a short distance down the slope before

its force was exhausted, while the thicker ice on either side advanced

farther before it was melted.

_Glacial Deposits._

Before especial reference is made to the drift of this particular

region, it will be well to consider the character of drift deposits in

general. When the ice of the continental glacier began its motion, it

carried none of the stony and earthy debris which constitute the drift.

These materials were derived from the surface over which the ice moved.

From the method by which it was gathered, it is evident that the drift

of any locality may contain fragments of rock of every variety which

occurs along the route followed by the ice which reached that locality.

Where the ice had moved far, and where there were frequent changes in

the character of the rock constituting its bed, the variety of materials

in the drift is great. The heterogeneity of the drift arising from the

diverse nature of the rocks which contributed to it is _lithological

heterogeneity_--a term which implies the commingling of materials

derived from different rock formations. Thus it is common to find pieces

of sandstone, limestone, quartzite, granite, gneiss, schist, etc.,

intimately commingled in the drift, wherever the ice which produced it

passed over formations of these several sorts of rock. Lithological

heterogeneity is one of the notable characteristics of glacial

formations.

Another characteristic of the drift is its _physical heterogeneity_. As

first gathered from the bed of moving ice, some of the materials of the

drift were fine and some coarse. The tendency of the ice in all cases

was to reduce its load to a still finer condition. Some of the softer

materials, such as soft shale, were crushed or ground to powder, forming

what is known in common parlance as clay. Clayey (fine) material is

likewise produced by the grinding action of ice-carried bowlders upon

the rock-bed, and upon one another. Other sorts of rock, such as soft

sandstone, were reduced to the physical condition of sand, instead of

clay, and from sand to bowlders all grades of coarseness and fineness

are represented in the glacial drift.

[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,

PL. XXXV.

Cut in drift, showing its physical heterogeneity.]

Since the ice does not assort the material which it carries, as water

does, the clay, sand, gravel and bowlders will not, by the action of the

ice, be separated from one another. They are therefore not stratified.

As left by the ice, these physically heterogeneous materials are

confusedly commingled. The finer parts constitute a matrix in which the

coarser are embedded.

Physical heterogeneity (Plate XXXV), therefore, is another

characteristic of glacial drift. It is not to be understood that the

proportions of these various physical elements, clay, sand, gravel, and

bowlders, are constant. Locally any one of them may predominate over any

or all the others to any extent.

Since lithological and physical heterogeneity are characteristics of

glacial drift, they together afford a criterion which is often of

service in distinguishing glacial drift from other surface formations.

It follows that this double heterogeneity constitutes a feature which

can be utilized in determining the former extension of existing

glaciers, as well as the former existence of glaciers where glaciers do

not now exist.

Another characteristic of glacial drift, and one which clearly

distinguishes it from all other formations with which it might be

confounded, is easily understood from its method of formation. If the

ice in its motion holds down rock debris upon the rock surface over

which it passes with such pressure as to polish and striate the

bed-rock, the material carried will itself suffer wear comparable to

that which it inflicts. Thus the stones, large and small, of glacial

drift, will be smoothed and striated. This sort of wear on the

transported blocks of rock, is effected both by the bed-rock reacting on

the bowlders transported over it, and by bowlders acting on one another

in and under the ice. The wear of bowlders by bowlders is effected

wherever adjacent ones are carried along at different rates. Since the

rate of motion of the ice is different in different parts of the

glacier, the mutual abrasion of transported materials is a process

constantly in operation. A large proportion of the transported stone and

blocks of rock may thus eventually become striated.

From the nature of the wear to which the stones are subjected when

carried in the base of the ice, it is easy to understand that their

shapes must be different from those of water-worn materials. The latter

are rolled over and over, and thus lose all their angles and assume a

more or less rounded form. The former, held more or less firmly in the

ice, and pressed against the underlying rock or rock debris as they are

carried slowly forward, have their faces planed and striated. The

planation and striation of a stone need not be confined to its under

surface. On either side or above it other stones, moving at different

rates, are made to abrade it, so that its top and sides may be planed

and scored. If the ice-carried stones shift their positions, as they may

under various circumstances, new faces will be worn. The new face thus

planed off may meet those developed at an earlier time at sharp angles,

altogether unlike anything which water-wear is capable of producing. The

stone thus acted upon shows a surface bounded by planes and more or less

beveled, instead of a rounded surface such as water wear produces. We

find, then, in the shape of the bowlders and smaller stones of the

drift, and in the markings upon their surfaces, additional criteria for

the identification of glacier drift (Plate XXXVI).

The characteristics of glacial drift, so far as concerns its

constitution, may then be enumerated as, (1) its lithological, and (2)

physical heterogeneity; (3) the shapes, and (4) the markings of the stones

of the drift. In structure, the drift which is strictly glacial, is

unstratified.

In the broadest sense of the term, all deposits made by glacier ice are

_moraines_. Those made beneath the ice and back from its edge constitute

the _ground moraine_, and are distinguished from the considerable

marginal accumulations which, under certain conditions, are accumulated

at or near the margin. These marginal accumulations are _terminal

moraines_. Associated with the moraines which are the deposits of the

ice directly, there are considerable bodies of stratified gravel and

sand, the structure of which shows that they were laid down by water.

This is to be especially noted, since lack of stratification is

popularly supposed to be the especial mark of the formations to which

the ice gave rise.

[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,

PL. XXXVI.

Glaciated stones, showing both form and striae. (Matz.)]

These deposits of stratified drift lie partly beyond the terminal

moraine, and partly within it. They often sustain very complicated

relations both to the ground and terminal moraines.

The drift as a whole is therefore partly stratified and partly

unstratified. Structurally the two types are thoroughly distinct, but

their relations are often most complex, both horizontally and

vertically. A fuller consideration of these relations will be found on a

later page.

_The Ground Moraine._

The ground moraine constitutes the great body of the glacial drift.

_Bowlder clay_, a term descriptive of its constitution in some places,

and _till_, are other terms often applied to the ground moraine. The

ground moraine consists of all the drift which lodged beneath the ice

during its advance, all that was deposited back from its edge while its

margin was farthest south, and most of that which was deposited while

the ice was retreating. From this mode of origin it is readily seen that

the ground moraine should be essentially as widespread as the ice

itself. Locally, however, it failed of deposition. Since it constitutes

the larger part of the drift, the characteristics already enumerated

as belonging to drift in general are the characteristics of the

till. Wherever obstacles to the progress of the ice lay in its path,

there was a chance that these obstacles, rising somewhat into the

lower part of the ice, would constitute barriers against which debris in

the lower part of the ice would lodge. It might happen also that the

ice, under a given set of conditions favoring erosion, would gather a

greater load of rock-debris than could be transported under the changed

conditions into which its advance brought it. In this case, some part of

the load would be dropped and over-ridden. Especially near the margin of

the ice where its thickness was slight and diminishing, the ice must

have found itself unable to carry forward the loads of debris which it

had gathered farther back where its action was more vigorous. It will be

readily seen that if not earlier deposited, all material gathered by the

under surface of the ice would ultimately find itself at the edge of the

glacier, for given time enough, ablation will waste all that part of the

ice occupying the space between the original position of the debris, and

the margin of the ice. Under the thinned margin of the ice, therefore,

considerable accumulations of drift must have been taking place while

the ice was advancing. While the edge of the ice sheet was advancing

into territory before uninvaded, the material accumulated beneath its

edge at one time, found itself much farther from the margin at another

and later time. Under the more forcible ice action back from the margin,

the earlier accumulations, made under the thin edge, were partially or

wholly removed by the thicker ice of a later time, and carried down to

or toward the new and more advanced margin. Here they were deposited, to

be in turn disturbed and transported still farther by the farther

advance of the ice.

Since in its final retreat the margin of the ice must have stood at all

points once covered by it, these submarginal accumulations of drift must

have been made over the whole country once covered by the ice. The

deposits of drift made beneath the marginal part of the ice during its

retreat, would either cover the deposits made under the body of the ice

at an earlier time, or be left alongside them. The constitution of the

two phases of till, that deposited during the advance of the ice, and

that deposited during its retreat, is essentially the same, and there

is nothing in their relative positions to sharply differentiate them.

They are classed together as _subglacial till_.

Subglacial till was under the pressure of the overlying ice. In keeping

with these conditions of accumulation, the till often possesses a

firmness suggestive of great compression. Where its constitution is

clayey it is often remarkably tough. Where this is the case, the quality

here referred to has given rise to the suggestive name "hard pan." Where

the constitution of the till is sandy, rather than clayey, this firmness

and toughness are less developed, or may be altogether wanting, since

sand cannot be compressed into coherent masses like clay.

_Constitution._--The till is composed of the more or less comminuted

materials derived from the land across which the ice passed. The soil

and all the loose materials which covered the rock entered into its

composition. Where the ice was thick and its action vigorous, it not

only carried away the loose material which it found in its path, but,

armed with this material, it abraded the underlying rock, wearing down

its surface and detaching large and small blocks of rock from it. It

follows that the constitution of the till at any point is dependent upon

the nature of the soil and rock from which it was derived.

If sandstone be the formation which has contributed most largely to the

till, the matrix of the till will be sandy. Where limestone instead of

sandstone made the leading contribution to it, the till has a more

earthy or clayey matrix. Any sort of rock which may be very generally

reduced to a fine state of division under the mechanical action of the

ice, will give rise to clayey till.

The nature and the number of the bowlders in the till, no less than the

finer parts, depend on the character of the rock overridden. A hard and

resistant rock, such as quartzite, will give rise to more bowlders in

proportion to the total amount of material furnished to the ice, than

will softer rock. Shale or soft sandstone, possessing relatively slight

resistance, will be much more completely crushed. They will, therefore,

yield proportionately fewer bowlders than harder formations, and more

of the finer constituents of till.

The bowlders taken up by the ice as it advanced over one sort of rock

and another, possessed different degrees of resistance. The softer ones

were worn to smaller dimensions or crushed with relative ease and speed.

Bowlders of soft rock are, therefore, not commonly found in any

abundance at great distances from their sources. The harder ones yielded

less readily to abrasion, and were carried much farther before being

destroyed, though even such must have suffered constant reduction in

size during their subglacial journey. In general it is true that

bowlders in the till, near their parent formations, are larger and less

worn than those which have been transported great distances.

The ice which covered this region had come a great distance and had

passed over rock formations of many kinds. The till therefore contains

elements derived from various formations; that is, it is lithologically

heterogeneous. This heterogeneity cannot fail to attract the attention

of one examining any of the many exposures of drift about Baraboo at

road gradings, or in the cuts along the railway. Among the stones in the

drift at these exposures are limestone, sandstone, quartzite, diabase,

gabbro, gneiss, granite, schist, and porphyry, together with pieces of

flint and chert.

Such an array may be found at any of the exposures within the immediate

vicinity of Devil's lake. To the north, and a few miles to the south of

the Baraboo ranges, the quartzite from these bluffs, and the porphyry

from the point marked h in Plate II, are wanting, though other

varieties of porphyry are present. The ice moved in a general

west-southwest direction in this region, and the quartzite in the drift,

so far as derived from the local formation, is therefore restricted to a

narrow belt.

The physical heterogeneity may be seen at all exposures, and is

illustrated in Plate XXXV. The larger stones of the drift are

usually of some hard variety of rock. Near the Baraboo ranges, the local

quartzite often predominates among the bowlders, and since such

bowlders have not been carried far, they are often little worn. Away

from the ranges, the bowlders are generally of some crystalline rock,

such as granite and diabase. Bowlders of these sorts of rock are from a

much more distant source, and are usually well worn.

In general the till of any locality is made up largely of material

derived from the formations close at hand. This fact seems to afford

sufficient warrant for the conclusion that a considerable amount of

deposition must have gone on beneath the ice during its movement, even

back from its margin. To take a concrete illustration, it would seem

that the drift of southeastern Wisconsin should have had a larger

contribution than it has of material derived from Canadian territory, if

material once taken up by the ice was all or chiefly carried down to its

thinned edge before deposition. The fact that so little of the drift

came from these distant sources would seem to prove that a large part of

the material moved by the ice, is moved a relatively short distance

only. The ice must be conceived of as continually depositing parts of

its load, and parts which it has carried but a short distance, as it

takes up new material from the territory newly invaded.

In keeping with the character of till in general, that about Devil's

lake was derived largely from the sandstone, limestone and quartzite of

the immediate vicinity, while a much smaller part of it came from more

distant sources. This is especially noticeable in the fine material,

which is made up mostly of the comminuted products of the local rock.

_Topography._--The topography of the ground moraine is in general the

topography already described in considering the modification of

preglacial topography effected by ice deposition. As left by the ice,

its surface was undulating. The undulations did not take the form of

hills and ridges with intervening valleys, but of swells and depressions

standing in no orderly relationship to one another. Undrained

depressions are found in the ground moraine, but they are, as a rule,

broader and shallower than the "kettles" common to terminal moraines.

It is in the broad, shallow depressions of the ground moraine that many

of the lakes and more of the marshes of southeastern Wisconsin are

located.

The rolling, undulating topography characteristic of ground moraines is

well shown about the City of Baraboo and between that point and the

lake, and at many less easily designated points about Merrimac.

In thickness the ground moraine reaches at least 160 feet, though its

average is much less--too little to obliterate the greater topographic

features of the rock beneath. It is, however, responsible for many of

the details of the surface.

_Terminal Moraines._

The marginal portion of the ice sheet was more heavily loaded--certainly

more heavily loaded relative to its thickness--than any other. Toward

its margin the thinned ice was constantly losing its transportive power,

and at its edge this power was altogether gone. Since the ice was

continually bringing drift down to this position and leaving it there,

the rate of drift accumulation must have been greater, on the average,

beneath the edge of the ice than elsewhere.

Whenever, at any stage in its history, the edge of the ice remained

essentially constant in position for a long period of time, the

corresponding submarginal accumulation of drift was great, and when the

ice melted, the former site of the stationary edge would be marked by a

broad ridge or belt of drift, thicker than that on either side. Such

thickened belts of drift are _terminal moraines_. It will be seen that a

terminal moraine does not necessarily mark the terminus of the ice at

the time of its greatest advance, but rather its terminus at any time

when its edge was stationary or nearly so.

From the conditions of their development it will be seen that these

submarginal moraines may be made up of materials identical with those

which constitute the ground moraine, and such is often the case. But

water arising from the melting of the ice, played a much more

important role at its margin than farther back beneath it. One result of

its greater activity may be seen in the greater coarseness which

generally characterizes the material of the terminal moraine as compared

with that of the adjacent ground moraine. This is partly because the

water carried away such of the finer constituents as it was able to

transport, leaving the coarser behind. Further evidence of the great

activity of water near the margin of the ice is to be seen in the

relatively large amount of assorted and stratified sand and gravel

associated with the terminal moraine.

Such materials as were carried on the ice were dropped at its edge when

the ice which bore them melted from beneath. If the surface of the ice

carried many bowlders, many would be dropped along the line of its edge

wherever it remained stationary for any considerable period of time. A

terminal moraine therefore embraces (1) the thick belt of drift

accumulated beneath the edge of the ice while it was stationary, or

nearly so; and (2) such debris as was carried on the surface of the ice

and dumped at its margin. In general the latter is relatively

unimportant.

At various stages in its final retreat, the ice made more or less

protracted halts. These halting places are marked by marginal moraines

of greater or less size, depending on the duration of the stop, and the

amount of load carried.

A terminal moraine is not the sharp and continuous ridge we are wont to

think it. It is a belt of thick drift, rather than a ridge, though it is

often somewhat ridge-like. In width, it varies from a fraction of a mile

to several miles. In the region under consideration it is rarely more

than fifty feet high, and rarely less than a half mile wide, and a ridge

of this height and width is not a conspicuous topographic feature in a

region where the relief is so great as that of the Devil's lake region.

_Topography of terminal moraines._--The most distinctive feature of a

terminal moraine is not its ridge-like character, but its peculiar

topography. In general, it is marked by depressions without outlets,

associated with hillocks and short ridges comparable in dimensions to

the depressions. Both elevations and depressions are, as a rule, more

abrupt than in the ground moraine. In the depressions there are many

marshes, bogs, ponds and small lakes. The shapes and the abundance of

round and roundish hills have locally given rise to such names as "The

Knobs," "Short Hills," etc. Elsewhere the moraine has been named the

"Kettle Range" from the number of kettle-like depressions in its

surface. It is to be kept in mind that it is the association of the

"knobs" and "kettles," rather than either feature alone, which is the

distinctive mark of terminal moraine topography.

[Illustration: Fig. 37.--Sketch of terminal moraine topography, on the

quartzite ridge east of Devil's lake. (Matz.)]

The manner in which the topography of terminal moraines was developed is

worthy of note. In the first place, the various parts of the ice margin

carried unequal amounts of debris. This alone would have caused the

moraine of any region to have been of unequal height and width at

different points. In the second place, the margin of the ice, while

maintaining the same _general_ position during the making of a moraine,

was yet subject to many minor oscillations. It doubtless receded to some

slight extent because of increased melting during the summer, to advance

again during the winter. In its recession, the ice margin probably did

not remain exactly parallel to its former position. If some parts

receded more than others, the details of the line of its margin may have

been much changed during a temporary retreat. When the ice again

advanced, its margin may have again changed its form in some slight

measure, so as to be parallel neither with its former advanced position,

nor with its position after its temporary retreat. With each successive

oscillation of the edge, the details of the margin may have altered, and

at each stage the marginal deposits corresponded with the edge. There

might even be considerable changes in the edge of the ice without any

general recession or advance, as existing glaciers show.

It was probably true of the margin of the American ice sheet, as of

existing glaciers, that there were periods of years when the edge of the

ice receded, followed by like periods when it remained stationary or

nearly so, and these in turn followed by periods of advance. During any

advance, the deposits made during the period of recession would be

overridden and disturbed or destroyed.

If the ice were to retreat and advance repeatedly during a considerable

period of time, always within narrow limits, and if during this

oscillation the details of its margin were frequently changing, the

result would be a complex or "tangle" of minor morainic ridges of

variable heights and widths. Between and among the minor ridges there

would be depressions of various sizes and shapes. Thus, it is conceived,

many of the peculiar hillocks and hollows which characterize terminal

moraines may have arisen.

Some of the depressions probably arose in another way. When the edge of

the ice retreated, considerable detached masses of ice might be left

beyond the main body. This might be buried by gravel and sand washed out

from the moraine. On melting, the former sites of such blocks of ice

would be marked by "kettles." In the marginal accumulations of drift as

first deposited, considerable quantities of ice were doubtless left.

When this melted, the drift settled and the unequal settling may have

given rise to some of the topographic irregularities of the drift.

_The terminal moraine about Devil's lake._--On the lower lands, the

terminal moraine of the Devil's lake region has the features

characteristic of terminal moraines in general. It is a belt of thick

drift varying in width from half a mile or less to three-quarters of a

mile or more. Its surface is marked by numerous hills and short ridges,

with intervening depressions or "kettles." Some of the depressions among

the hills contain water, making ponds or marshes, though the rather

loose texture of the drift of this region is not favorable to the

retention of water. The moraine belt, as a whole, is higher than the

land on either side. It is therefore somewhat ridge-like, and the small,

short hills and ridges which mark its surface, are but constituent parts

of the larger, broader ridge.

Approached from the west, that is from the driftless side, the moraine

on the lower lands is a somewhat prominent topographic feature, often

appearing as a ridge thirty, forty or even fifty feet in height.

Approached from the opposite direction, that is, from the ground

moraine, it is notably less prominent, and its inner limit wherever

located, is more or less arbitrary.

[Illustration: Fig. 38.--Cut through the terminal moraine just east of

Kirkland, partially diagrammatic.]

A deep, fresh railway cut in the moraine southeast of Devil's lake

illustrates its complexity of structure, a complexity which is probably

no greater than that at many other points where exposures are not seen.

The section is represented in Fig. 38. The stratified sand to the right

retains even the ripple-marks which were developed when it was

deposited. To the left, at the same level, there is a body of _till_

(unstratified drift), over which is a bed of stoneless and apparently

structureless clay. In a depression just above the clay with till both

to the right and left, is a body of loam which possesses the

characteristics of normal loess. It also contains calcareous

concretions, though no shells have been found. This occurrence of loess

is the more noteworthy, since loess is rarely found in association with

drift of the last glacial epoch.[7]

[7] An account of loess in connection with the drift of the

last glacial epoch is given in the _Journal of Geology_, Vol.

IV, pp. 929-987. For a general account of loess, see Sixth

Annual Report of U.S. Geological Survey.

_The moraine on the main quartzite range._--In tracing the moraine over

the greater quartzite range, it is found to possess a unique feature in

the form of a narrow but sharply defined ridge of drift, formed at the

extreme margin of the ice at the time of its maximum advance. For fully

eleven miles, with but one decided break, and two short stretches where

its development is not strong, this unique marginal ridge separates the

drift-covered country on the one hand, from the driftless area on the

other. In its course the ridge lies now on slopes, and now on summits,

but in both situations preserves its identity. Where it rests on a

plain, or nearly plain surface, its width at base varies from six to

fifteen rods, and its average height is from twenty to thirty feet. Its

crest is narrow, often no more than a single rod. Where it lies on a

slope, it is asymmetrical in cross section (see Fig. 39), the shorter

slope having a vertical range of ten to thirty-five feet, and its longer

a range of forty to one hundred feet. This asymmetrical form persists

throughout all that portion of the ridge which lies on an inclined

surface, the slope of which does not correspond with the direction of

the moraine. Where it lies on a flat surface, or an inclined surface

the slope of which corresponds in direction with the course of the ridge

itself, its cross section is more nearly symmetrical (see Fig. 40). In

all essential characteristics this marginal ridge corresponds with the

_End-Moraene_ of the Germans.

[Illustration: Fig. 39.--Diagrammatic cross-section of the marginal

ridge as it occurs on the south slope of the Devil's Nose. The slope

below, though glaciated, is nearly free from drift.]

[Illustration: Fig. 40.--Diagrammatic cross-section of the marginal

ridge as it appears when its base is not a sloping surface.]

For the sake of bringing out some of its especially significant

features, the ridge may be traced in detail, commencing on the south

side of the west range. Where the moraine leaves the lowlands south of

the Devil's nose, and begins the ascent of the prominence, the marginal

ridge first appears at about the 940-foot contour (f, Plate XXXVII).

Though at first its development is not strong, few rods have been passed

before its crest is fifteen to twenty feet above the driftless area

immediately to the north (see Fig. 39) and from forty to one hundred

feet above its base to the south, down the slope. In general the ridge

becomes more distinct with increasing elevation, and except for two or

three narrow post-glacial erosion breaks, is continuous to the very

summit at the end of the nose (g). The ridge in fact constitutes the

uppermost forty or forty-five feet of the crest of the nose, which is

the highest point of the west range within the area shown on the map.

Throughout the whole of this course the marginal ridge lies on the south

slope of the nose, and has the asymmetrical cross section shown in Fig.

39. Above (north of) the ridge at most points not a bowlder of drift

occurs. So sharply is its outer (north) margin defined, that at many

points it is possible to locate it within the space of less than a yard.

At the crest of the nose (g) the marginal ridge, without a break,

swings northward, and in less than a quarter of a mile turns again to

the west. Bearing to the north it presently reaches (at h) the edge of

the precipitous bluff, bordering the great valley at the south end of

the lake. Between the two arms of the loop thus formed, the surface of

the nose is so nearly level that it could have offered no notable

opposition to the progress of the ice, and yet it failed to be covered

by it.

[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V.,

PL. XXXVII.

Topographic map (contour interval 100 feet) of a small area about

Devil's lake, taken from the Baraboo sheet of the United States

Geological Survey. Each contour line connects points of the same

elevation, and the figures upon them give the heights above sea level.

Where contour lines lie close together, they indicate steep slopes.]

In the great valley between the nose and the east bluff, the marginal

ridge does not appear. In the bottom of the valley the moraine takes on

its normal form, and the slopes of the quartzite ridges on either hand

are much too steep to allow any body of drift, or loose material of any

sort, to lodge on them.

Ascending the east bluff a little east of the point where the drift

ridge drops off the west bluff, the ridge is again found (at i) in

characteristic development. For some distance it is located at the edge

of the precipitous south face of the bluff. Farther on it bears to the

north, and soon crosses a col (j) in the ridge, building it up many

feet above the level of the bed-rock. From this point eastward for about

three miles the marginal ridge is clearly defined, the slopes about

equal on either side, and the crest as nearly even as the topography of

the underlying surface permits. The topographic relations in this part

of the course are shown in Fig. 40.

At k, this marginal ridge attains its maximum elevation, 1,620 feet.

At this great elevation, the ridge turns sharply to the northwest at an

angle of more than 90 deg.. Following this direction for little more than

half a mile, it turns to the west. At some points in this vicinity the

ridge assumes the normal morainic habit, but this is true for short

distances only. Farther west, at l, it turns abruptly to the northeast

and is sharply defined. It here loops about a narrow area less than

sixty rods wide, and over half a mile in length, the sharpest loop in

its whole course. The driftless tract enclosed by the arms of this loop

is lower than the drift ridge on either hand. The ice on either side

would need to have advanced no more than thirty rods to have covered the

whole of it.

From the minor loop just mentioned, the marginal ridge is continued

westward, being well developed for about a mile and a half. At this

point the moraine swings south to the north end of Devil's lake, loses

the unique marginal ridge which has characterized its outer edge

across the quartzite range for so many miles, and assumes the topography

normal to terminal moraines. At no other point in the United States, so

far as known to the writers, is there so sharply marked a marginal ridge

associated with the terminal moraine, for so long a distance.

From Plate II it will be seen that the moraine as a whole makes a

great loop to the eastward in crossing the quartzite range. From the

detailed description just given of the course of the marginal ridge, it

will be seen that it has three distinct loops; one on the Devil's nose

(west of g, Plate XXXVII); one on the main ridge (west of k)

and a minor one on the north side of the last (southwest of m). The

first and third are but minor irregularities on the sides of the great

loop, the head of which is at k.

The significant fact in connection with these irregularities in the

margin of the moraine is that each loop stands in a definite relation to

a prominence. The meaning of this relation is at once patent. The great

quartzite range was a barrier to the advance of the ice. Acting as a

wedge, it caused a re-entrant in the advancing margin of the glacier.

The extent and position of the re-entrant is shown by the course of the

moraine in Plate II. Thus the great loop in the moraine, the head of

which is at k, Plate XXXVII, was caused by the quartzite range itself.

The minor loops on the sides of the major are to be explained on the

same principle. Northeast of the minor loop on the north side of the

larger one (m) there are two considerable hills, reaching an elevation

of nearly 1,500 feet. Though the ice advancing from the east-northeast

overrode them, they must have acted like a wedge, to divide it into

lobes. The ice which reached their summits had spent its energy in so

doing, and was unable to move forward down the slope ahead, and the

thicker bodies of ice which passed on either side of them, failed to

unite in their lee (compare Figs. 34 and 35). The application of the

same principle to the loop on the Devil's nose is evident.

_Constitution of the marginal ridge._--The material in the marginal

ridge, as seen where erosion has exposed it, is till, abnormal, if at

all, only in the large percentage of widely transported bowlders which

it contains. This is especially true of the surface, where in some

places 90 per cent. of the large bowlders are of very distant origin,

and that in spite of the fact that the ice which deposited them had just

risen up over a steep slope of quartzite, which could easily have

yielded abundant bowlders. In other places the proportion of foreign

bowlders is small, no more than one in ten. In general, however,

bowlders of distant origin predominate over those derived close at hand.

_The slope of the upper surface of the ice at the margin._--The marginal

ridge on the south slope of Devil's nose leads to an inference of

especial interest. Its course lies along the south slope of the nose,

from its summit on the east to its base on the west. Throughout this

course the ridge marks with exactness the position of the edge of the

ice at the time of its maximum advance, and its crest must therefore

represent the slope of the upper surface of the ice at its margin.

The western end of the ridge (f, Plate XXXVII) has an altitude of 940

feet, and its eastern end (g) is just above the 1,500-foot contour.

The distance from the one point to the other is one and three-fourths

miles, and the difference in elevation, 560 feet. These figures show

that the slope of the ice along the south face of this bluff was about

320 feet per mile. This, so far as known, is the first determination of

the slope of the edge of the continental ice sheet _at its extreme

margin_. It is to be especially noted that these figures are for the

extreme edge of the ice only. The angle of slope back from the edge was

doubtless much less.

_Stratified Drift._

While it is true that glacier ice does not distinctly stratify the

deposits which it makes, it is still true that a very large part of the

drift for which the ice of the glacial period was directly or indirectly

responsible is stratified. That this should be so is not strange when it

is remembered that most of the ice was ultimately converted into running

water, just as the glaciers of today are. The relatively small portion

which disappeared by evaporation was probably more than counterbalanced,

at least near the margin of the ice, by the rain which fell upon it.

It cannot be considered an exaggeration, therefore, to say that the

total amount of water which operated on the drift, first and last, was

hardly less than the total amount of the ice itself. The drift deposited

by the marginal part of the ice was affected during its deposition, not

only by the water which arose from the melting of the ice which did the

depositing, but by much water which arose from the melting of the ice

far back from the margin. The general mobility of the water, as

contrasted with ice, allowed it to concentrate its activities along

those lines which favored its motion, so that different portions of the

drift were not affected equally by the water of the melting ice.

All in all it will be seen that the water must have been a very

important factor in the deposition of the drift, especially near the

margin of the ice. But the ice sheet had a marginal belt throughout its

whole history, and water must have been active and effective along this

belt, not only during the decadence of the ice sheet, but during its

growth as well. It is further to be noted that any region of drift stood

good chance of being operated upon by the water after the ice had

departed from it, so that in regions over which topography directed

drainage after the withdrawal of the ice, the water had the last chance

at the drift, and modified it in such a way and to such an extent as

circumstances permitted.

_Its origin._--There are various ways in which stratified drift may

arise in connection with glacier deposits. It may come into existence by

the operation of water alone; or by the co-operation of ice and water.

Where water alone was immediately responsible for the deposition of

stratified drift, the water concerned may have owed its origin to the

melting ice, or it may have existed independently of the ice in the form

of lakes. When the source of the water was the melting ice, the water

may have been running, when it was actively concerned in the deposition

of stratified drift; or it may have been standing (glacial lakes and

ponds), when it was passively concerned. When ice co-operated with water

in the development of stratified drift the ice was generally a passive

partner.

_Glacial drainage._--The body of an ice sheet during any glacial period

is probably melting more or less at some horizons all the time, and at

all horizons some of the time. Most of the water which is produced at

the surface during the summer sinks beneath it. Some of it may congeal

before it sinks far, but much of it reaches the bottom of the ice

without refreezing. It is probable that melting is much more nearly

continuous in the body of a moving ice sheet than at its surface, and

that some of the water thus produced sinks to the bottom of the ice

without refreezing. At the base of the ice, so long as it is in

movement, there is doubtless more or less melting, due both to friction

and to the heat received by conduction from the earth below. Thus in the

ice and under the ice there must have been more or less water in motion

throughout essentially all the history of an ice sheet.

If it be safe to base conclusions on the phenomena of existing glaciers,

it may be assumed that the waters beneath the ice, and to a less extent

the waters in the ice, organized themselves to a greater or less degree

into streams. For longer or shorter distances these streams flowed in

the ice or beneath it. Ultimately they escaped from its edge. The

subglacial streams doubtless flowed, in part, in the valleys which

affected the land surface beneath the ice, but they were probably not

all in such positions.

The courses of well-defined subglacial streams were tunnels. The bases

of the tunnels were of rock or drift, while the sides and tops were of

ice. It will be seen, therefore, that their courses need not have

corresponded with the courses of the valleys beneath the ice. They may

sometimes have followed lines more or less independent of topography,

much as water may be forced over elevations in closed tubes. It is not

to be inferred, however, that the subglacial streams were altogether

independent of the sub-ice topography. The tunnels in which the water

ran probably had too many leaks to allow the water to be forced up over

great elevations. This, at least, must have been the case where the ice

was thin or affected by crevasses. Under such circumstances the

topography of the land surface must have been the controlling element

in determining the course of the subglacial drainage.

When the streams issued from beneath the ice the conditions of flow were

more or less radically changed, and from their point of issue they

followed the usual laws governing river flow. If the streams entered

static water as they issued from the ice, and this was true where the

ice edge reached the sea or a lake, the static water modified the

results which the flowing waters would otherwise have produced.

_Stages in the history of an ice sheet._--The history of an ice sheet

which no longer exists involves at least two distinct stages. These are

(1) the period of growth, and (2) the period of decadence. If the latter

does not begin as soon as the former is complete, an intervening stage,

representing the period of maximum ice extension, must be recognized. In

the case of the ice sheets of the glacial period, each of these stages

was probably more or less complex. The general period of growth of each

ice sheet is believed to have been marked by temporary, but by more or

less extensive intervals of decadence, while during the general period

of decadence, it is probable that the ice was subject to temporary, but

to more or less extensive intervals of recrudescence. For the sake of

simplicity, the effects of these oscillations of the edge of the ice

will be neglected at the outset, and the work of the water accompanying

the two or three principal stages of an ice sheet's history will be

outlined as if interruptions in the advance and in the retreat,

respectively, had not occurred.

As they now exist, the deposits of stratified drift made at the edge of

the ice or beyond it during the period of its maximum extension present

the simplest, and at the same time most sharply defined phenomena, and

are therefore considered first.

_Deposits Made by Extraglacial Waters During the Maximum Extension of

the Ice._

The deposits made by the water at the time of the maximum extension of

the ice and during its final retreat, were never disturbed by subsequent

glacier action. So far as not destroyed by subsequent erosion, they

still retain the form and structure which they had at the outset. Such

drift deposits, because they lie at the surface, and because they are

more or less distinct topographically as well as structurally, are

better known than the stratified drift of other stages of an ice sheet's

history. Of stratified drift made during the maximum extension of the

ice, and during its final retreat, there are several types.

_A. At the edge of ice, on land._--If the subglacial streams flowed

under "head," the pressure was relieved when they escaped from the ice.

With this relief, there was diminution of velocity. With the diminution

of velocity, deposition of load would be likely to take place. Since

these changes would be likely to occur at the immediate edge of the ice,

one class of stratified drift deposits would be made in this position,

in immediate contact with the edge of the ice, and their form would be

influenced by it. At the stationary margin of an ice sheet, therefore,

at the time of its maximum advance, ice and water must have co-operated

to bring into existence considerable quantities of stratified drift.

The edge of the ice was probably ragged, as the ends of glaciers are

today, and as the waters issued from beneath it, they must frequently

have left considerable quantities of such debris as they were carrying,

against its irregular margin, and in its re-entrant angles and marginal

crevasses. When the ice against which this debris was first lodged

melted, the marginal accumulations of gravel and sand often assumed the

form of kames. A typical kame is a hill, hillock, or less commonly a

short ridge of stratified drift; but several or many are often

associated, giving rise to groups and areas of _kames_. Kames are often

associated with terminal moraines, a relation which emphasizes the fact

of their marginal origin.

So far as the superficial streams which flowed to the edge of the ice

carried debris, this was subject to deposition as the streams descended

from the ice. Such drift would tend to increase the body of marginal

stratified drift from subglacial sources.

Marginal accumulations of stratified drift, made by the co-operation of

running water and ice, must have had their most extensive development,

other things being equal, where the margin of the ice was longest in one

position, and where the streams were heavily loaded. The deposits made

by water at the edge of the ice differ from those of the next

class--made beyond the edge of the ice--in that they were influenced in

their disposition and present topography, by the presence of ice.