Valley Glaciers and Ice Caps. -The most common conception of glaciers is derived from mountain regions like the Alps or Canadian Rockies. Such glaciers occupy mountain valleys, and there should be no confusion when they are sometimes called mountain or Alpine glaciers, and at other times valley glaciers.
In colder latitudes ice of the same nature may form a broad sheet covering a perfectly plain surface, or it may cover a rough surface so deep that the highest hills, or even low mountains, are obscured. Such a sheet is also commonly spoken of as a glacier, but it should be distinguished as a continental glacier or ice cap. Such an ice cap now covers Greenland, having an area of at least 300,000 square miles. A similar cap covers perhaps several million square miles of the Antarctic continent. The ice of such continental glaciers may be at places several thousand feet deep.
Neve-. - All glacier ice is derived from snow where the annual snowfall exceeds melting, and is not, like ordinary ice, formed directly by the freezing of water. The gradual change from snow to ice may be observed even in the latitude of Ohio. When a deep pile of snow is protected from melting until late spring, it may be observed that the once flaky snow has become granular like a mass of small pellets. Those who have tried to make snowballs in the late summer from the snow on high mountains which may have lasted over many seasons, will recall the same granular condition still more pronounced. Each grain is a pellet of pure solid ice. By a process not here explained, these pellets get larger as time goes on, some no doubt disappearing and giving their substance to others. In this condition the material is called 'neve', a term taken from those parts of the Alps where French is spoken. The term neve, while used in this sense to designate the material, is used also in a slightly different sense to designate the great snow fields which cover the mountain slopes above the level of the glacier.
Glacier Ice.-Where the neve is very deep, as in a mountain valley, its own weight presses the lower granules very close together, compacting them into a solid mass. This is helped by melting at the surface, the resulting water trickling down and freezing in the pores. By these two processes neve becomes glacier ice. Those who visit glaciers should not fail to pour a little ink on a piece of the ice. The ink follows the lines or seams where the granules meet and thus outlines their form. In most glaciers their sizes range from that of peas to that of cherries, but they may be as large as oranges.
Thus at the upper end of the mountain valley the ice is confined to the bottom where the pressure is great. All above is neve, and at the surface ordinary snow. As the same material creeps very slowly down the valley, consuming much time in its progress, it changes more and more to ice. The greater warmth of the lower valley may destroy the snow cover, so that in late summer many glaciers carry no snow at their lower ends (Fig. 42).
Movement and Crevasses.-The forward movement of an Alpine glacier is in part (though not wholly) a mere matter of sliding down grade. The grade may change from place to place. The valley also widens and narrows alternately and its course is far from straight. On these accounts the glacier cracks or forms crevasses. In fact, it is customary to define the limit of the neve (snowfield) and the beginning of the glacier by the place where crevassing begins. These crevasses may open a fraction of an inch or ten to twenty feet, and may be so abundant as to make the surface impossible to cross, especially after melting has widened their tops and narrowed the intervening ice blocks.
Former Ice Sheets of North America.-In a late geological period, called Pleistocene or Glacial, snowfall greatly exceeded melting over the northern part of North America, and glacier ice accumulated to so great a depth that it spread southward over Ohio and the states west of it (Fig. 43). To some extent, no doubt, the general slope of the continent was southward, but to some extent also, the southward movement was a mere spreading out, due to piling up at the north.
Thickness.-The total thickness of the ice in Canada may have been a few thousand feet. It was doubtless a few hundred feet thick in the Miami Valley. Generally speaking, it began to grow thinner by excessive melting after crossing the Great Lakes. Over northern Canada, at least, the snowfall exceeded melting.
Movement Over an Uneven Surface. -The surface of the ice necessarily sloped in the direction indicated by its movement, but this did not prevent the glacier from crossing great valleys like those of the Great Lakes, a hundred or more miles in width and a thousand or more feet in depth. The ice filled these valleys, and must therefore have been 1,000 or more feet thicker in the basin of Lake Superior than on its margins. It was able to climb the slope to the southern margin because the slope of its upper surface was always in that direction. The ice therefore moved up hill in much the same way that water does when flowing over a dam.
The rate of ice movement may have been a few inches or a few feet per day. In moving over the land, the ice was obliged to accommodate itself to the uneven surface. In any one position the ice was fitted to hills and valleys as a die fits a mold, but the same surfaces would not fit when the ice had moved forward to a new position. Thus the ice was continually obliged to fit itself to new topographic forms. This it did partly by breaking and refreezing. Ice also melts more readily when subjected to pressure, hence where it pressed unusually hard against an obstacle there was a tendency to melt a hollow in the ice to fit the obstacle, and where a gap was left between ice and earth water might accumulate and often freeze.
But the accommodation was not all on the side of the ice. The hills and valleys were obliged to compromise. The former were worn down where ice was thick and heavy, but not in southern Ohio where the ice was relatively thin and rapidly wasting. Valleys were in many cases filled or partly filled, either temporarily by stagnant ice or more permanently by rock debris. There were also other and less simple ways by which the ice was continually molded and remolded to the surface over which it moved.
Advance and Retreat of Front.-As already stated the ice is constantly moving forward, melting meantime both on the top and at the edges. By reason of this continual advance of ice from a colder region, the glacier is able to survive indefinitely in a region where melting greatly exceeds the snowfall. At length, however, its front reaches a position where the rate of melting at the edge equals the rate of advance. Beyond this it cannot advance. So long as this balance continues the edge remains stationary. But the balance is liable to disturbance. The climate may become temporarily warmer or colder or the volume of on-coming ice may be greater or less. In either case the edge of the ice shifts. This is important in the area near the Ohio as will be seen later.
Lobes.-Important effects are produced at the edge of the ice by large valleys where these trend in the direction of the ice movement, as was the case with the Miami, Little Miami, and Mill Creek valleys. Regarding the surface of the ice as essentially a plane, it is clear that in such cases the ice is much thicker in the large valleys than over the intervening divides. Moreover, its movement, where thin, is more interfered with by friction, and it is more easily broken up. It is easily understood that thin ice melts quicker than thick ice. It comes about, therefore, that large valleys thus situated are often occupied by glacial ice when the intervening higher ground is bare. This is probably much more apt to be the case during the retreat of the ice front than during its advance. The more important effects of glaciation may, therefore, be expected in the great valleys. This is abundantly illustrated near Ohio River as described later.
If valleys trend transversely to the ice movement they are more apt to be filled with rock debris if not too large.
Glacial Erosion.- Several allusions have been made above to rock debris carried by the ice. The larger part of the geologic importance of glaciers in the United States consists in their relation to such material. Glaciers bear very heavily on the earth below and in their onward movement abrade a large amount of material, first the mantle rock and later the solid rock. This applies to those regions where the glacier is accumulating, or at least maintaining its full thickness and weight; not to such regions as southwestern Ohio where the ice was rapidly wasting and unable to carry forward the debris it had already loosened, much less to erode the fresh bed rock. Kelleys Island in Lake Erie is well known for its beautiful illustrations of what the ice can do in grinding away, polishing, striating and fluting the solid limestone (Fig. 44). At that place all mantle rock must first have been swept away, but near the Ohio, not only was the bed rock not eroded, but much of the old pre-glacial mantle remains.
Debris on the Ice.-Alpine glaciers derive much debris from the steep side slopes of their valleys from which rocks roll down on the ice. The very nature of a continental glacier shuts this out. Only an exceptional mountain, called in Greenland a 'nunatak', may rise above the ice. Its steep slopes, made steeper by the erosion of the ice at its foot, may drop debris on the ice which stretches out as a long dark train in the lee of the mountain, but aside from this exceptional case there is little or no opportunity for any rock material to get on the surface of the glacier. Continental glaciers, therefore, carry little or nothing on top. An exception may be found near their edges where dust has been blown off, or where melting has carried away all the clean ice of the upper portion and is at work on the debris-filled lower portion. The rock material thus set free accumulates on the surface and tends to prevent deeper melting.
Debris in the Ice.-Much of the material which glaciers erode from the surface is merely rubbed along between the ice and the earth, but much of it also is carried up into the ice, especially that which is eroded from hilltops. A great deal also is incorporated into the glacier by the ice freezing around it, .and some of that ,which falls on top goes down into crevasses. In almost any mass of drift a number of stones will be found which have thus been carried in the ice at its base and scratched or "striated" by rubbing over others or over the solid rock of the bed. Similar scratches on bed rock are very useful in showing the direction of movement.
How Deposited.-The ways described above, in which glaciers obtain and carry their loads of rock material, suggest the ways in which it may be deposited. Perhaps the most important of all deposits, at least in the United States, are those made beneath the ice and known as ground moraine. So long as a glacier continues in a climate favorable to accumulation, its thickness and freight and the power with which it rubs on its bed are kept up, and little material can come to rest beneath it. This was the case in northern Canada. Before going very far north of the Great Lakes the traveler sees bare rock becoming more and more frequent. Even in northeastern Minnesota is a considerable area which was severely glaciated, but on which almost no deposit was left. When a glacier gets thinner and lighter it becomes unable to drag along beneath it all the load it has hitherto carried. The load therefore begins to lodge and form a sheet of ground moraine (Fig. 45). This is generally the case south of the latitude of Lake Superior. The change is a gradual one, but in the main the continental glaciers were not accumulating but wasting within the limits of the United States.
But the glacier is not to be thought of as invading the United States with a burden of detritus and depositing just enough at every stage to make up for the loss in carrying power. The glacier was not only depositing everywhere in Ohio, but everywhere taking up new burden. Therefore the detritus deposited at any given place consisted largely of material derived from no great distance. The proportions from farther north are smaller in proportion to the distance. It is fair to estimate that, of the drift thus deposited on the southern border of any county in southern Ohio, one-half the. material came from that county itself, or at any rate from the last two counties passed over by the ice. Farther north, where the glacier was more vigorous, the proportion of material from long distances is greater.
Ground moraine is made up only in part of drift carried beneath the ice. The continental glacier, where wasting, is melting not only at the top but also at the bottom, thus setting free a part of the load carried in the ice. When set free by bottom melting this also joins the ground moraine.( See note 12)
Material thus left by the ice generally contains much clay. Sand and stones are embedded in the clay without assortment. The stones may be of any size up to huge bowlders. Some of them may be scratched or striated.
Local Character.- From the principles here stated it will be apparent that the character of the drift at any given place reflects to a considerable extent that of the rocks just passed over. This pertains not only to the stones embedded in the drift, but to the finer matrix as well. It is sandy where the underlying rock is sandstone, clayey where the rocks have been either shale or such as to yield clay on decomposition, and calcareous where the rocks have been limestone.
The thickness of ground moraine may be anything from a sprinkling to more than 100 feet. Throughout much of northeastern United States the total thickness of the drift is greater than this, but made up of several sheets of ground moraine laid down in different epochs. Very great thicknesses of drift (say 400 to 800 feet) are apt to be the result of other processes described below under the head of terminal moraine.
Topography of Ground Moraine. - The topography of ground moraine is apt to show the effect of the ice movement over it. The result will depend on the amount of glacial erosion and the amount of deposition. There is always a tendency to obliterate former valleys or to fill them irregularly, causing shallow undrained basins which retain lakes and swamps. If the glacier be weak and the deposit thin, as was the case in this area, the pre-glacial valleys may be little changed, and there may be neither lakes nor swamps. A very thick deposit may reduce the country almost to a plain as around Columbus, O., and the resulting streams may have nothing to do with pre-glacial valleys. Elsewhere, as in central New York, pre-glacial valleys may be obstructed and lakes result. In all cases there is a lack of angular topographic features.
Ridge at the Ice Front.-Deposits made at the edge of the ice cover much less area than that of the ground moraine but are locally much more prominent. When the on-coming glacier melts at the edge as fast as it advances, the debris which it carries in, on, and below the ice, comes to rest. If the edge of the ice remains on the same line for many years this material accumulates in a ridge called terminal moraine. In exceptional cases such ridges are several hundred feet high and only a fraction of a mile wide, but smaller altitudes and greater widths are more common.
Modes of Deposit.- The most evident factor in the growth of terminal moraine is the mere dumping of rock debris as the ice which carried it melts. Another factor is the depositing of material carried to the front by glacial waters. These, either on or under the ice, are apt to run in narrow channels, but spread out at the edge of the ice losing their velocity- and depositing their load in corners or recesses, or merely against the edge of the ice. When the latter melts away these deposits are left as mere piles of sand and gravel
(kames) within the zone of terminal moraine of which they form a part. Sometimes the ice front, in its alternate recession and advance, pushes up into a steep ridge the material which lies in the way of its advance. A less simple and noticeable, but very important factor, is the lodging of debris below the ice as it rapidly thins by melting both above and below within the last mile or very few miles from the edge. This is only an accentuation of the same process already described in the deposit of ground moraine, and illustrated by figure 45, but the thinning of the glacier near its edge is so rapid that the accumulation below begins somewhat abruptly to thicken at a rapid rate, thus building up a slope on the back of the ridge of terminal moraine. When pronounced, such a slope is commonly regarded as the back slope of the terminal moraine, though it is not sharply differentiated from the ground moraine.
Topography of Terminal Moraine.-All these processes, except the last, tend to give the terminal moraine a different style of surface from the ground moraine. The surface of the latter is dominated by the effect of the ice in passing over it. Its whole surface must be thought of as conforming to a moving sheet of ice. The surface of the land (omitting subsequent erosion) must also be thought of as the undersurface of a moving ice sheet. On the other hand, the dominating factor in the topography of the terminal moraine is gravity acting on mere fortuitous dumps or piles of drift. This statement does not apply to the kind of back slope just described, but even on such a slope the hills and hollows are less orderly and more abrupt than in the ground moraine because of the abundance of the deposit and the diminishing power of the ice. Fig. 46, though representing the work of water rather than of ice, gives a good idea of morainic topography.
Drift Sheets Without Terminal Moraine. - The description here given of the making of a terminal moraine implies a glacier (or ice-sheet) moving with considerable vigor and transporting much material to the limits of its advance. Most of the continental glaciers in North America built little or no terminal moraine. Such is the case with the first one which visited this area and crossed the Ohio.
Apparently in such cases the movement was feeble and the ice thinned very gradually to its edge, so that all its load was spread out in a sheet before the edge was reached. The same effect would be produced if the ice were pushed forward to its extreme limit one or more times and then remained stagnant, or, if the ice itself continued to advance, the edge may have begun at once to retreat by excessive melting. In any case there was no constantly advancing debris-laden sheet with its edge remaining stationary.
The sheet which later covered the northern part of the area, while making no well defined ridges of terminal moraine here, made some deposits with the typically morainic topography. These are described in a later chapter.
Only a part of the debris carried by the ice is dropped at once into its place when the ice melts. . The balance is carried farther by the water which results from the melting ice or which falls regularly as rain. This glacial debris, rehandled and redeposited by water is built into various topographic features according to the slopes of the land where the glacier ends.
Outwash Plains.- If the glacier ends on flat ground, each stream which runs down its sloping surface to its edge will lose its power on reaching the edge and deposit its load in an alluvial fan. If the load be at all abundant such alluvial fans will soon grow laterally until they merge and all together form but a single gentle slope away from the ice, it may be for five or ten miles or even farther. This is called an outwash plain and is not well illustrated in this area for the simple reason that there was little flat ground where the glaciers ended.
Lake Deposits.-If the glacier ended on a surface of valleys and ridges in such a way that the local drainage was toward the ice, local lakes were formed, one shore of which was ice. In these lakes deltas were formed, or, in some cases, the lakes were entirely filled with sediment. Banklick Creek in Colerain Township, Hamilton County, was thus dammed. So also were the West Fork of Mill Creek two miles southwest of Glendale, and Sharon Creek one and one-half miles above Sharonville. In all these cases deep deposits of lacustrine silt were made. The several streams have now cut down through these sediments, exposing their beds to view. They have been in part removed by erosion; their remnants constitute terraces.
Valley Trains.-If the glacier ended on a surface having valleys sloping from the ice, the detritus was carried down stream. All such streams flowing from the ice are overloaded with detritus, so that it forms bars and shoals and natural levees, all these being forms which attend the filling of a valley by alluvium. Such valleys become filled or partly filled with alluvium which makes a flat flood plain or valley floor (Fig. 47). This process is well illustrated in the region here described where the great valleys leading south were filled to the level now represented by terraces. The most prominent of these is that on which the business section of Cincinnati is built. Such outwash is called a valley train and consists of sand and gravel.
