The preglacial lower Mohawk River — Changes of the Ice Age in the lower valley — Outlets of the postglacial Mohawk into Lake Albany — The geology of Cohoes Falls — The Cohoes Mastodon and Pleistocene animals of the Mohawk Valley.
The Preglacial Lower Mohawk
The Mohawk Valley below Schenectady forms an interesting study of conditions before and after the coming of the glacier. The Valley, west of Schenectady, has all the signs of an old river valley, while, eastward from that city, the course of the river and its watershed has a decidedly modern character. The natural conclusion is that the Mohawk, from Schenectady westward, generally follows its ancient preglacial course while the present lower Mohawk channel is comparatively recent, as geological time is reckoned.
The Mohawk River and its watershed is peculiar in that a great part of its upper basin (from Little Falls westward) and its lower section (from Schenectady to Cohoes) are developments of glacial action and have been added to the main central Valley since preglacial times. Moreover the main eastward flowing river, by glacial action, has annexed a westward flowing upper stream ("Rome River," from Little Falls to Rome) and changed its course from west to east, a fact which gives the Mohawk's history a further and particular interest in connection with the story of the great glacier which overspread our Valley for thousands of years. In preglacial times this Rome River was one of the upper headwaters of the Dundas River, corresponding to the Mississippi of today. There are few existing streams which have undergone such peculiar changes as those here described in connection with the history of the Mohawk River.
The location of the buried channel of the preglacial Mohawk, eastward from near Schenectady, has not been definitely determined. The United States Board of Engineers of Deep Waterways in 1899, surveyed the Mohawk River to ascertain its availability as a ship canal channel. Borings made during this survey show that "near South Schenectady, the bedrock stands at an elevation of 320 and then abruptly falls off to 210 feet beneath the valley of the Poentic Kill, a southern tributary to the Mohawk basin about three-fourths of a mile north, beyond which rock was not reached." The borings were made only to 190 feet sea level. Along a line extending from the bank of the river, at a point three-fifths of a mile west of the Schenectady waterworks to a point near South Schenectady, no rock was struck. These borings indicate that there is a depression in the bedrock in the locality north of South Schenectady. It is possible that the preglacial Mohawk flowed in this direction and found its outlet following, in a general way, the channel of the present Normanskill. It is this route that was favored by the Deep Waterways Survey, as the ship channel course from the Mohawk above South Schenectady to the Hudson River on the southern limits of Albany. If such a channel existed in preglacial times, a large part of the present site of Albany then lay in the Mohawk watershed, just as a considerable portion of the city of Cohoes, at the present outlet of the stream, is in the Mohawk's drainage basin.
The Preglacial Hudson
Geological evidences point to a preglacial stream flowing southward from the Ballston Lake channel into the Mohawk at about the present site of Schenectady. In the same period, a southward flowing stream drained the Saratoga-Round Lake depression which marks an ancient channel of the Hudson, as shown in the accompanying map. The foregoing conditions were those pertaining to the lower Mohawk River course during preglacial times.
[Series of 6 maps illustrating drainage along the Mohawk River in various periods.]
The Postglacial Lower Mohawk Channel
The development of the stages of the lower postglacial Mohawk channel is clearly shown by the accompanying maps. Together with this condensed description, they are taken from the "Glacial Geology of the Schenectady Quadrangle" (New York State Museum Bulletin 154) and "Glacial Geology of the Cohoes Quadrangle" (New York State Museum Bulletin 215-216) by James H. Slotter.
Following the withdrawal of the southern edge of the ice sheet, the waters of Lake Albany spread over the region from the present Hudson to and west of Schenectady, flooding the lower Mohawk River channel. The flooded Mohawk then began to build up the present delta between Schenectady and Albany, now known as the "sand plains." It may have been at this time that the preglacial Mohawk channel was filled up or the channel may have been previously obstructed by glacial drift as was the case with the Sacandaga River at Northampton and the West Canada Creek at Prospect. The size and character of these Mohawk delta deposits on the bed of Lake Albany show that this lake existed for a long period of geologic time, as did the great Iroquois-Mohawk or Iromohawk which carried the waters of the Great Lakes basin eastward to Lake Albany. The large volume of water and its accompanying silt, then carried by the great Iromohawk, also accounts for this great delta then laid down by our river's ancient waters. This condition is shown in the first of the maps showing the postglacial outlet of the Mohawk into Lake Albany.
Subsidence of Lake Albany
The time came when Lake Albany's waters began to subside by the lowering of the outlet southward to the sea. The delta southeast of Schenectady then emerged as a land surface and the Mohawk's current became confined within a channel conforming with the basin near Schenectady. This basin exists today in the present broad river channel there filled with islands (of which Van Slyck's is the largest) and now crossed by the Great Western Gateway Bridge. From there the Mohawk had two channels to Lake Albany. It was then that the river found a spillway eastward across the rocks below Aqueduct and the eroding of the present lower channel of the Mohawk River began. The greatest flow of the waters of the then great Mohawk at first found their main outlet northward through the Ballston channel and discharged into the lowered Lake Albany near East Line. The present lower Mohawk, because of its narrow channel at Aqueduct, lowered its bed more rapidly than the wider Ballston channel. This greater flow of water through the Aqueduct channel cut out the present gorge east of that place and the flooded waters thence swept eastward in a broad path through the Lake Albany deposits. Thus was formed the broad depression extending southeastward, from Crescent to the islands in the mouth of the Mohawk.
As the waters of Lake Albany fell, the lake, greatly narrowed, began to take on the character of a broad river and the strong currents from the north and the Iromohawk (which then still had its upper channel into the Hudson at present Mechanicsville) cut deeply into the sediments still filling the midportion of the general Hudson valley. As Lake Albany waters continued to fall and as the Mohawk deepened its channel through the rock barrier at Aqueduct, the river's bed was scoured out to a point below the elevation of the Ballston channel and the divide was then formed between the Mohawk and Ballston Lake. All the Mohawk's waters then passing through its present lower channel, the stream cut a broad path through the deposits southeast of Crescent, laying bare the rock floor of the preglacial Hudson valley. Where the Mohawk flowed over the rock slope, which marked the rim of the ancient valley, a falls developed which, through recession, formed the present falls and gorge of the Mohawk at Cohoes.
Cohoes Falls
The previous description of the geology of the lower channel of the Mohawk shows that the Cohoes Falls are comparatively modern as reckoned in geological time, being postglacial in their origin and history. While Niagara Falls, another postglacial cataract, has receded seven miles since its beginning the falls of Cohoes have cut back a distance of only about 2,000 feet. This is due to the fact that only a part of the postglacial greater Mohawk's waters came through the present channel for a long period of time and also because of the character of the rock bed of the river in the region of the falls.
Says Stoller:
"The Mohawk River, from near Crescent to its mouth, has its bed on rock and, in the last three miles of its course, occupies a rock gorge marked by the falls at Cohoes. A dam has been built across the upper portion of the gorge in order to supply power for the industries of Cohoes. The level of the water in the dam is 156 feet. From the dam to the falls the river has the character of a rapids descending 20 feet in the distance of three-fourths of a mile. It then falls 70 feet over a precipice of rock. The water does not descend abruptly as a vertical sheet but flows over the steeply inclined rock declivity. This slope corresponds in general to the dip of the rocks but, in the middle portion of the falls, the rocks have been worn and broken so that the angle of the fall is less than that of the dip and the falling water has the character of a cascade. At about one-third of the width of the river, from the left (north) bank, there is a projecting mass of smoothed rock over which very little water passes. At the inner side of this mass the volume of falling water is greater than elsewhere and, at the base, there is a deep pool of water occupying a depression worn in the rocks.
"Below the falls the river flows on a bed of jagged rock but with slower current, descending, in the course of a mile, from the level of 60 feet at the foot of the falls to 51 feet at the head of the lower rapids, where the river divides into three streams (popularly called the Sprouts of the Mohawk), forming the islands on the floor of the Hudson Valley. In the summer, when the river is low, most of the water below the falls is confined to a deepwater channel, which extends from the deepwater pool at the falls."
The slow recession of the falls is explained by the fact that the bed of the stream consists of indurated shales and sandstone dipping steeply eastward. The direction of flow follows the dip and the water wear, on the rocks at the front of the falls precipice, is over the surface of their bedding planes. At the summit of the falls, the water flows over the edges of the rock and here the wear is naturally more rapid. Due to this more rapid erosion of the rock strata on their upper ends than on their faces the angle of curve of the rock face of the falls is greater than that of the dip of the strata. This greater wear at the summit and at the base, due to the force of the falling waters, gives the Cohoes Falls their unusual and characteristic rounded appearance. This is fully shown when the rock structure of the falls is largely uncovered during the low water of summer. The great postglacial Mohawk, as well as the smaller stream of the later Quaternary period, has. drilled a number of potholes in the rock bed of the stream, both above and below the present cataract.
The Cohoes Falls is both interesting and picturesque. When viewed, at high water, from the south bank at the base of the gorge, the cataract is truly impressive and justifies the somewhat sensational tales of early travelers concerning Cohoes.
In the general public consideration of Mohawk Valley geology, the point of view seems to have been largely sectional and not one embracing the Valley as a whole, which is the intent of this chapter. For the trifling reason that the falls of Cohoes lie outside of the imaginary lines which are supposed to define the six Mohawk Valley Counties, these interesting great falls of the Mohawk have had little attention in works written for the people of the middle and upper sections of the Mohawk Valley. Naturally the Cohoes Falls region is of equal importance with all other areas of the watershed of the Mohawk and this picturesque cataract and its interesting geological history give the lower Mohawk channel unusual interest.
* * *
Cohoes Mastodon
Probably the two most famous deceased animals of New York State are two representatives of the elephant family — Barnum's famous Jumbo, the skeleton of which stands in the Museum of Natural History in New York City, and the Cohoes mastodon, which makes its present home in the midst of the fine collections of the New York State Museum in the Education Building at Albany. This celebrated Mastodon Americanus rivals the famous Devonian tree trunks of Gilboa, the oldest in the world, as an attraction to students of geology and paleontology visiting the Museum. Both have brought fame to the Mohawk Valley as being products of our world-noted region. Here, too, we can see our old-time four-legged citizen both with his clothes off and on — making an imposing exhibit as a mastodonic skeleton, while his lifelike counterpart, standing nearby arrayed in his shaggy coat of fur, presents a most formidable appearance. To the Eskimo, who paddled the glacial lakes of the Mohawk Valley, thousands of years ago, such a vision as this meant food and raiment for an entire long postglacial winter.
The name mastodon is a word derived from the Greek and means "nipple teeth" from the peculiar nipples on the molars of the mastodon, seven of which were on each side of the upper and lower jaw. The character of these teeth is midway between those of herbivorous animals and those of the mammoth and its descendant, the elephant. Bones of the mammoth have been found with those of the mastodon in New York State. This mammoth Elephas Americanus is a different specie from the Asiatic and European mammoth Elephas primigenius, which is closely allied to the Indian elephant. The American mammoth's remains are much scarcer than those of the mastodon. The food of both species probably principally consisted of the leaves and branches of pine, willows, birches and other hardy trees. They roamed America and Europe in herds, much like the buffalo herds of the plains, prior to 1870. The stone age men of Europe hunted them and pictured them on bone surfaces and on the walls of their caves. Doubtless the Eskimo people, who here followed the retreating ice sheet at the close of the Ice Age, hunted them in the woods, swamps, lake and river borders of postglacial North America. Both the mastodon and the mammoth were heavily built creatures, having twice the weight of the modern elephant. We associate the word mammoth with colossal objects, because of this extinct creature's heavy, stocky bulk. The name, however, comes from the Tartar word "mamma", meaning earth, as this people thought the animals burrowed in the earth, because of their long tusks and from the fact that their fossil bones were brought to light below the surface of the ground.
The Cohoes mastodon is probably the most noted of his species in America, as this was one of the first specimens in which the skeleton was found largely intact and because it was one of the earliest mounted specimens. The incidents of its discovery also gave it wide publicity as it was uncovered in one of the deepest potholes ever excavated in the Mohawk River bed. The site of this pothole is shown on the relief model sketch of the Cohoes Falls neighborhood, which locates it on the upper rock level of the ancient Iroquois-Mohawk River, which site must date back to a period when the Mohawk first began to cut back its present falls, thus assuring the Cohoes mastodon a very respectable antiquity.
In September, 1866, workmen were excavating for a foundation for an addition to the Harmony Mills at Cohoes. In these digging operations the men came upon the lower jaw and single foot bone lying on a ledge of rock at the side of a filled-in pothole. Further excavation, to a depth of fifty feet, brought to light the principal parts of the skeleton lying on a bed of clay and broken slate above a layer of waterworn pebbles, with which the powerful water currents had drilled this great pothole. This lower layer was ten feet thick and showed that the pothole was old before the mastodon was here entombed. The fill in the pothole above these bones consisted of fifty feet of muck, peaty soil, fragments of tree limbs, rotten beaver-gnawed wood, washed here by the flood of the great Iromohawk. On top of this was a layer of artificial fill, the work of modern man.
The Cohoes mastodon is the only actual specimen found within the limits of the Mohawk Valley and its location was on its very eastern edge. There is reason to believe, however, that these animals ranged the more level parts of our Valley just as they did other parts of the State.
The remains of contemporary Pleistocene mammals are frequently found mingled with those of the mastodon and mammoth in New York State. From a list of the following, thus found, we can form some idea of the animals which roamed the shores of the postglacial Mohawk: fossil bear of the character of the grizzly; black bear; fox; seal; fossil giant beaver "Castoroides", an animal with the bulk of a modern black bear and the proportions of a modern beaver, with which remains it is frequently associated; fossil peccary; deer; elk, "fossil stag"; caribou; moose; American bison, which probably roamed a great part of the present State; fossil horse, of a type larger than our domesticated animal.
Some of these animals may have belonged to the period of the Eskimo-like people, whose remains are found in our State and Valley, and probably were hunted by these first of our known aborigines.