DIRECTION OF MOTION OF THE GLACIAL ICE
27. The Glacial Ice Sheet; Its Extent and Centers of Distribution
As already indicated in the preceeding lines, the greatest transporting agency in moving fragments of rock from Canada, and from the nearer areas north of us, southward, toward Dayton, was the ice, in the form of a glacier of such large size that usually it is referred to as a continental glacier.
This continental glacier covered most of the northern part of the North American continent excepting certain parts of northern Alaska. It originated in various areas from which the ice spread until it formed a more or less continuous sheet.
Even after the ice sheet was continuous there were certain areas where [p.75] the accumulation of ice appears to have been greatest, and which served as centers of distribution from which the ice sheet spread in a more or less radiate direction southward. The so-called Labrador center occupied the broad peninsula between Hudson bay and the coast of Labrador. From this center the ice spread southward to Long Island, New Jersey, Pennsylvania, Ohio, Indiana and Illinois, reaching the northern border of Kentucky between Cincinnati and Louisville at various localities.
[Map: Map of North America during the glacial ice age, showing the Cordilleran, Keewatin and Labradorean centers of accumulation from which the ice moved. Note that the continental ice sheet reached only the southern margin of Alaska. (After Chamberlain.)]
The Keewatin center occupied the broad territory between Hudson bay and the rockies. From this center the ice spread southward to northern Missouri, eastern Kansas, Nebraska and South Dakota, and the northern parts of North Dakota and Montana. The Labrador and Keewatin [p. 76]
ice sheets met, and at times more or less overlapped, along a line extending along the upper parts of the present Mississippi river and the axis of Lake Winnipeg.
The Cordilleran center occupied the northern Rockies and the ranges farther westward. From this center the ice sheet spread into northern Washington and Idaho, and northwestern Montana.
28. The Glacial Ice Lobes
From each of these centers of distribution the ice sheet spread southward, but to various distances, producing an irregular ice front. As was to be expected, the ice sheet spread farther southward along the lowlands than over the more elevated areas, forming large lobes.
Thus, during one of the later stages of the ice sheet, the Miami lobe extended down the Miami river to within a short distance of Cincinnati, occupying the territory between Xenia, in Ohio, and Connersville, in Indiana.
The Scioto lobe extended as far south as Hillsboro and Chillicothe, and spread laterally from xenia on the west to Lancaster, Newark, and Mt. Vernon on the east. The main axis of this lobe lies along the Scioto river. In a similar manner the Maumee valley, Saginaw bay, and Lake Michigan formed the axes of glacial lobes.
29. Direction of Motion of Glacial Ice
If the pebbles in the gravel ridges south of Dayton originated from localities north of Dayton, in some cases from localities even as far north as Canada, and if these pebbles were transported not only by the streams forming the gravel ridges, but also, during their earlier stages of travel, by the very slowly moving glacial ice, then not only the ridge-building streams, but also the transporting glacial ice sheet must have moved southward.
By far the largest part of the transporting was done by the moving ice sheet. The streams, at the southern margins of the ice sheet, however, probably did most of the work of rounding the rock fragments into peb- [p.77] bles, and certainly did all of the work of sorting the pebbles and sand into layers.
30. The Direction of Motion of the Glacial Ice Near Dayton Indicated by Glacial Striae
The direction of motion of the glacial ice frequently is indicated locally also by long, straight scratches or striae cut into the uppermost layers of the continuous sheets of rock.
In former years, when the numerous quarries in the vicinity of Dayton were worked extensively, large surfaces of rock were often laid bare by the removal of the overlying clay, gravel, and soil, preparatory to the wedging or blasting out of the underlying rock. In these cases, the exposed surface of the topmost layer of limestone, directly beneath the clay or soil, was found smoothed as though by some enormous lanning machine. Crossing the smoothed surfaces of the rock were numerous scratches or striae. Some of these were large enough to be called grooves, but by far the larger number were quite narrow and of very slight depth. Many of these finer striae were not likely to attract attention except when closely observed by a keen investigator.
The most striking feature about these striae was their straightness and their comparative uniformity of direction. Not only in the same quarry, but sometimes over wide areas, the general direction of these striae was quite uniform.
From this it is evident that all these striae owed their existence to the same general cause. The same agency must have smoothed all of these rock surfaces.
Many of these striae could be followed locally across the entire area of exposure, sometimes for distances of more than 100 feet. On close examination many of the striae were seen to cross each other, but the great majority maintained about the same general trend.
In the area between Centerville, 9 miles south of Dayton, and Troy, 18 miles north of Dayton, the striae have a southeasterly course. At the large quarry a third of a mile northwest of the railroad station east of Centerville, the direction is S 47 degrees E. See page 89. This expression means [p.78]
[Photo: Glacial striae running 10 degrees east of south across the upper surface of an exposure of Springfield limestone at the eastern margin of a quarry three-quarters of a mile west of Kingsville, along a short east and west road north of the Eaton pike.]
that the direction followed by the striae deviates 47 degrees toward the east from a direct south line. At the abandoned limestone quarries along the western edge of Beavertown, or Dean postoffice, the direction is S 27 degrees E. Half a mile south of the Soldiers’ Home grounds, on the eastern side of the West Carrollton pike, the direction is S 35 degrees E. At the road crossing one mile northwest of Kingsville, a mile north of the Eaton pike, the direction is S 25 degrees E. Three quarters of a mile west of Kingsville, at the quarry visible north of the Eaton pike, the direction is S 10 degrees E. This quarry, located north of a small east and west road, presents the best exposure of glacial striae at present readily accessible in the vicinity of Dayton. At the top of a small gully in the Miller woods, west of the Union road and north of the Eaton pike, the direction is S 15 degrees E.
A mile and a quarter east of Taylorsburg, and a quarter of a mile west of Shoup’s lime kiln, the direction of the striae is S 27 degrees E. At [p.79] this locality the top of the Dayton limestone formerly was well exposed in a large open quarry along the east and west road, a quarter of a mile west of the old Troy pike. The removal of very little soil would again result in a remarkabley fine exhibition of striae. At the top of the falls, in the woods half a mile southwest of Charleston, glacial striae vary between S 20 degrees E and S 30 degrees E at different point located within a short distance of each other. Along the edge of the bluff overlooking the Miami river, two miles north of Tippecanoe city, striae running S 28 degrees E were found, but a single layer of rock, northeast of the Crystal spring, at this locality, showed striae running S 45 degrees E. This rock layer was 18 feet above the railroad level, and sufficiently far below the flatlands back of the bluff to show the influence of topography upon the direction of these striae. It is quite evident that in this case the striae run parallel to the course of the neighboring channel of the Miami river. Ordinarily, the topography does not appear to have any great influence upon the direction of these striae.
Directly east of West Milton, seven and a half miles west of Tippecanoe city, the direction of the striae is S 30 degrees E. Along the road across the hill, about a mile west of Simms station, the striae run 15 degrees east of south.
31. The Spreading of the Ice Toward the Margin of the Miami Lobe
All of the localities here mentioned occur along the eastern side of the Miami lobe of the glacial ice sheet. Along this eastern side the general direction of motion of the ice evidently was about 30 degrees east of south. Nearer the middle parts of the lobe, for instance, a few miles northeast of Hamilton and northeast of Oxford, the direction of the glacial striae was more southerly, and along the western margin of the lobe, as New Paris, in Ohio, and also north and west of Richmond, in Indiana, their course was southwest.
A study of all the striae found within the area covered by the Miami lobe suggests that along the central axis of this lobe the general direction [p.80] of flow was southward, and from this central axis the ice spread out toward the southeast and southwest, toward the lateral margins of the lobe.
Farther north, where the Miami lobe merged into the general body of the continental ice sheet, the flow of the ice appears to have been more from the northeast. Apparently that part of the ice which farther south became sufficiently separated from the ice front on the east and west to be distinguished as the Miami lobe, crossed the central part of Lake Huron, with the adjacent parts of the southern peninsula of Michigan and Ontario, and flowed in a general southwesterly direction as far as Celina, in Ohio, gradually taking a more southerly course farther southward. See diagram on page 83.
This direction of flow would bring toward Dayton the granites, gneisses, schists, and quartzites from the areas north of Lake Huron. Rocks from areas distinctly east and west of this axis of flow should be rare or absent. For instance, in the vicinity of Dayton there should be no ice transported rocks from areas east of a line connecting Xenia, Springfield, Urbana, Bellefontaine, and Kenton, and rocks from areas east of Tiffin and Sandusky probably also should be rare if not entirely absent. For similar reasons, rocks from Indiana and the western half of the southern peninsula of Michigan should be rare or absent.
32. Glacial Striae Produced by Rocks Pushed Along by the Ice
The glacial striae evidently were produced by some agency moving in a general direction from north to south. In the vicinity of Dayton, on account of its location on the eastern side of the Miami lobe, the direction of the striae diverges toward the east or west from a southward direction are known.
The evidence of a general southward, rather than northward motion of the ice, in the vicinity of Dayton, sometimes is very interesting. Occasionally some fossil, or piece of chert or other hard substance in the rock will resist wear better than the surrounding softer limestone. In that case it may give rise to a small elevation above the general surface of the limestone, toward the south of which the rock is more or less protected from the [p.81] moving glacial ice, and hence less scratched for a short distance, while toward the north the glacial scratches may be very distinct.
In a similar manner, but on a much large scale, glacial scratches are likely to be less in evidence on rock slopes descending toward the south, toward some valley, than along rock slopes rising toward the south, especially if the amount of slope be considerable. Evidently the southward facing slopes were more sheltered, while the northward facing slopes received the impact of the moving ice more directly.
Resting upon the top of the smoothed and striated rock surfaces frequently are found small rocks, or even boulders, which themselves are smoothed and striated. This early suggested that the smoothing, and striating or scratching had been done by some agency which rubbed these rock fragments or boulders against the immediately underlying surface. Of course, this must have happened before the masses of clay, sand, and gravel, which now cover most of these smoothed rock surfaces, were present.
At first this agency was supposed to consist of icebergs, which were carried by currents of water in a general southerly direction across a land surface at that time supposed to have been covered with great bodies of water, the “Great Flood.” It soon was recognized, however, that such icebergs would become stranded readily on striking some shallow part of the sea bottom, and certainly would not move up and down over the irregular surfaces of the sea bottom with as little change in direction, especially where of large size and where crossing slopes of moderate degree compared with the thickness of the moving ice. Hence, the glacial origin of the striae or scratches soon was admitted.
33. The Continental Character of the Glacier Reaching Southern Ohio
The average traveler is familiar only with glaciers confined to comparatively narrow areas, terminating usually within a comparatively small number of miles in some narrow valley. Such are the glaciers of the Alps, famous for their ready accessibility and picturesque surroundings. These [p.82] are the glaciers of mountainous territories, originating at lofty altitudes and terminating at much lower levels. Owing to their mountainous origin they move down steep slopes and at comparatively rapid rates, considering the fact that glaciers are composed of material as rigid as ice.
[Map: Diagrammatic representation of successive positions of the ice border during the melting back of the continental glacier. The general direction of motion of the glacial ice is indicated by the dotted lines and arrows. By Frank Leverett and Frank B. Taylor, 1910.]
The conditions were very different with the continental glacier formerly covering almost all of the northern half of North America. It was of enormous proportions. The Labradorean, Keewatin, and Cordilleran ice sheets combined covered a total area of 4,000,000 square miles. From the central areas of dispersion, the Labradorean ice sheet extended 1,600 miles southwestward, as far as the southern parts of Illinois, while the [p.83] Keewatin ice sheet extended almost as far, 1,500 miles, to the central parts of Missouri. The only area of snow and ice at present in existence which can be compared with the continental ice sheet of former days is the Antarctic polar ice cap which also covers an area between three and four million square miles in extent. The next largest ice field still in existence is that which covers all of Greenland, excepting a narrow strip along the southwestern border. Its area has been estimated at 320,000 square miles. The largest glacier in Alaska, the Malaspina, covers an area 70 miles long, and between 20 and 25 miles wide, about as large as the state of Delaware. See map on page 76.
Even the lobes along the margin of the continental ice sheet were not confined along narrow valleys. The width of the Miami lobe, from Xenia on the east to Connersville on the west, was 60 miles. The width of the East-White river lobe, between Cambridge City on the east and a point beyond Indianapolis on the west, also approximates 60 miles. The Scioto lobe, on the contrary, between Urbana and Newark, attained a width of 80 miles. Numerous other lobes existed along the margin of the continental ice sheet, but these here cited are sufficient to indicate the enormous size of even the terminal lobes. See map on page 114.
Another great difference between the continental ice sheet and the Alpine glaciers is the comparative flatness of the territory over which most of the continental glacier moved. The center of dispersion of the Keewatin glacier is a low flat country, and yet from this center it moved from 800 to 1,000 miles westward, over what is now a westwardly rising plain, as far as the foothills of the Rockies. Although the center of dispersion of the Labradorean ice sheet lies on the Laurentide highlands, on the peninsula between Hudson bay and Labrador, nevertheless over a large part of its course, from the Ottawa river southwestward, it moved over comparatively flat country. South of the St> Lawrence basin and the Great Lakes the ice sheet also was obliged to rise, in order to pass over the watersheds limiting this basin southwards. In western Ohio, this water shed rises about 600 feet above the present level of Lake Erie, a shallow lake with an average depth of about 100 feet. Therefore the basal parts of the ice [p.84] sheet were obliged to rise at least 700 feet, south of Lake Erie, before reaching the lands sloping toward the Ohio river.
34. The Thickness of the Ice Sheet and the Slope of Its Upper Surface
The outward flow of the ice sheet from its centers of dispersion was determined much more by the amount of slope of the upper surface of the ice sheet than by the slope of the underlying land. There was a continual pressure from the areas in which the surface of the ice sheet attained the higher levels toward the areas where these surface levels were lower. In general, it may be assumed that the ice sheet was thickest where its surface attained the greatest elevation, but the point to be emphasized is that the flow of the ice was not so much from the point where it was the thickest to the point where it was the thinnest, as it was from the point of higher elevation toward a point of lower elevation. A thinner mass of ice at a higher elevation might force along a thicker mass of ice at a lower elevation, and in the last analysis the elevation of most importance is that along the upper surface of the ice, and not that along its basal portion.
On this account it would be interesting to know something about the former slope of the surface of the continental ice sheet. However, little is known. The greatest slope of the surface of the ice sheet probably was near its margin, where it was more or less rapidly melting away. At the edge of the ice sheet, near Baraboo, Wisconsin, where the ice moved along the side of a bold ridge, the average slope for the last one and three-fourths miles was 320 feet per mile, or a total of 560 feet in the entire distance. Several miles from the edge of the ice sheet, in certain parts of New Jersey and the adjacent parts of New York, the rate of slope of the upper surface of the ice was about 30 feet per mile. From these data, a thickness of 1,000 feet at a distance of 10 miles back from the ice front might be considered a moderate estimate. Farther back from the ice margin the slope of the upper surface of the ice sheet probably was less than 30 feet per mile, but even at the rate of 10 feet per mile the thickness of the ice sheet at its center of dispersion may have exceeded two and a half miles. Even if this num- [p. 85] ber should prove excessive, there is no doubt about the great thickness of the former continental ice sheet northward.
35. Rate of Motion of Glacial Ice
The most rapid rate of motion observed by Tyndall along the middle of one of the largest valley glaciers in the Alps, known as the Mer de Glace, was about three feet per day. The Muir Glacier, in Alaska, has a rate of motion of about seven feet per day. Both of these glaciers have steep descents. Where some of the glaciers of Greenland crowd into a narrow valley with a steep descent the rate of motion during summer time may exceed even 50 feet per day. On comparatively flat surfaces, during winter time, the rate of motion may be much slower. The average rate of motion of the inland ice of Greenland, during winter time and away from the areas of steep descent, is stated to be probably less than a foot per week.
It is probable that the rate of motion of the ice forming the continental ice sheet, during winter and away from the ice front, was comparable with this smallest number. At the margin, where the slope of the upper surface of the ice sheet was greatest, the rate of motion may have equalled one foot per day, or even more, locally.
36. Cause of Motion of Glacial Ice
Glacial ice does not move like a fluid, not even like a viscous fluid. When tar, under the influence of the heat of the sun, creeps down an inclined surface, it does not push any loose rock ahead of it, but flows over the rock and envelops it. Lava, in the same manner, does not push objects in front of it forward, but flows over them. Glacial ice, on the contrary, is rigid, and is pushed forward bodily. The rock embedded within the glacial ice is held rigidly and may be pushed forward like an engraving tool. The rigidity of ice is shown by the readiness with which it breaks or drops down to some lower level along its path.
While a downward slope of the land surface over which a glacier moves undoubtedly accelerates its rate of motion, it is evident, nevertheless, that this downward slope is not the sole factor determining the direction of flow [p. 86] of the ice. It has been noted already that the Keewatin ice sheet moved from a low-lying territory, along Hudson bay, westward for a distance of 800 to 1,000 miles along rising land toward the foot hills of the northern Rockies. In the same manner the Labradorean ice sheet moved from the valley of the St. Lawrence toward higher territory on approaching or crossing the present watersheds, southward. On the western slope of the Alleghenies, in northwestern Pennsylvania, the southern margin of the Labradorean ice sheet attained altitudes of nearly 2,000 feet above sea level, rising fully 1,400 feet above the present level of Lake Erie.
From this it may be seen that the ice moved as though thrust forward by some enormous force. This must have been a force acting not only in summer, but also in winter, although in winter the rate of motion of glacial ice is much retarded.
As already indicated on one of the preceding pages, this force probably was due to the difference in pressure produced by differences in level of the upper surface of enormous thicknesses of ice. This corresponded to the difference in pressure causing a flow in water from a point of higher level toward a point of lower elevation.
In order to account for the forward flow of such a rigid material as ice an ingenious theory has been invented which will appeal more readily to the student of physics than to the general reader. According to this theory the snow flakes, from which all glaciers start, sooner or later, under the influences of the sun, turn into granules of ice, somewhat like the granules of ice forming the crust on snow in winter time in our own climate. The accumulations of many snow falls interrupted by many days of sunshine result in the heaping up of enormous quantities of these ice granules, which, owing to their combined weight, tend to produce great pressure upon the bottom layers and to force them away. The looser ice granules actually may be pushed onward, to points of less pressure, which usually means down a valley, but those ice granules which adhered more firmly to their neighbors could be moved only by giving way at their points of contact. As is well know to students of physics, ice under pressure tend to melt, and if the melted water moves merely into some neighboring crevice, without being raised in temperature, it instantly freezes again. This is owing [p. 87] to the fact that pressure lowers the melting point of ice, and release of pressure permits refreezing.
Under these conditions, the pressure of the overlying weight of ice granules continually melts parts of the granules of ice at their points of contact, permitting an onward motion, the resulting water freezing in the intermediate spaces between the granules and adding to the rate of onward motion of the ice granules by the thrust produced by the expansion of the freezing water. The force of expanding ice during the freezing of water frequently is illustrated by the breaking of pitchers in winter.
Farther down the slope, where the ice granules have become more or less intergrown, the expanding force of freezing water, released by pressure at one point and expanding within a neighboring crevice, may be the chief factor in keeping up a continual thrust within the ice. Since this melting of the ice granules is due chiefly to pressure, at least when far below the surface of the glacier, it may take place not only in summer, but also in winter. This accounts in part for the streams of water which issue from beneath glaciers in great volume even during the coldest days of winter.
According to this theory, continual melting at one point of contact between ice granules and refreezing in a crevice at another point only a minute distance away may take place at great distances from the center of dispersion of a glacial ice sheet. Indeed, it has been suggested that the points of maximum thrust may be a considerable distance away from any one of these centers of dispersion.
While the force of expansion of the freezing water within any one of the minute crevices in the ice may not seem great, the accumulative effect of freezing water in countless numbers of these minute crevices over immense areas might be enormous; in fact, might be sufficient to account for the motion of glacial ice. This motion might be much accelerated by the presence of steep slopes, warm temperatures, an abundance of water, and of other factors, but the prime cause of motion would still be the melting of ice and the freezing of water where subjected to pressure and then released from the latter. Such a thrust due to freezing water might even force the margin of an ice sheet up to higher elevations, in opposition to the laws of gravity. [p. 88]
[Photo: Surface of Dayton limestone, in the Soldiers’ Home quarry, marked by glacial striae running S 35 E.]
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