Town Geology:
The Lesson of the Philadelphia Rocks
Studies of nature along the highways and among the
byways of a metropolitan town

by Angelo Helprin
Professor of Invertebrate Paleontology at, and Curator-in-Charge of, the Academy of Natural Sciences of Philadelphia

Philadelphia: Published by the Author. Academy of Natural Sciences, 1885

Chapter XI:
Philadelphia brick and cobble-stone:
A vision of arctic climates.
Pages 125-134

Almost everywhere throughout the built-up portion of the city where excavations of one kind or another are being made the pick and the spade disclose the presence of extensive deposits of clay and gravel. These can be readily seen exposed in the cuts of the various railroads leaving the city, in the foundation diggings for houses, and in all places where streams have effected permanent water-

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courses. Where the gravel and the clay occur together, the latter invariably overlies the former, proving that it was the last to form, and therefore that it is newest in date. In some places the clay has been completely washed away, and we have then only the gravel remaining. This formation of clay and gravel extends back from the Delaware River in an almost continuous sheet for an average distance of some four miles, and the clay itself can be traced up the slopes of the first line of hills to a height of some 150-200 feet. Where the clay is pure, or largely free from sand and gravel, as in the " Neck," or in the region about Nicetown, it is extensively used in the manufacture of bricks, a form of industry that is attested [PAGE 126] by the large number of brick-yards lying about. Scattered in greater or less quantities throughout this Philadelphia "brick clay," as the formation is frequently called, are pebbles and boulders of various sizes, mostly well-rounded, and indicating that they had been transported hither through the agency of water. In all cases they have the structure and composition of the rock-masses lying to the north, and we hence conclude that their source lies in that direction. Thus we have boulders of Cambrian quartzite, of Silurian sandrock, and of Triassic shale, formations that are to be met with along the line of the upper Delaware. Some of the boulders measure as much as three or four or more feet in length, but usually they are of very much smaller size. The gravel underlying the brick or boulder clays is of two kinds, a "red" and a "yellow" gravel, the former of which is the newer, and, consequently, the upper one when the two are in superposition. It owes its peculiar color to a ferruginous or iron clay in which the pebbles are imbedded, whose presence conduces to "packing," and renders the gravel eminently fit for the purposes of road-making. Although the predominating material in both the red and yellow gravel is approximately the same-white quartz or quartzite, and less abundantly sandstone, hornstone, and chert-the two can generally be distinguished from each other, apart from the differences presented by color, by the presence or absence of pebbles of red shale, which are very abundant in the former, and wholly wanting in the latter. The yellow gravel, too, is more distinctly fossiliferous, coral and shell-pebbles, indicative of the waste of some Silurian shore-line, being by no means uncommon. It will be observed in many places, as immediately back of the Biological Institute of the University of Pennsylvania, and in the open lots of the upper part [PAGE 127] of the city, that the clay is let down into a series of hollows in the red gravel, separating so many crests. Evidently, the water which held the clay in suspension must have, previous to depositing it, scooped out portions of the gravel, for in no other way can we account for this very singular disposition of the material. And this scooping could only have been effected by a sweeping current possessed of an oscillatory or wavy motion. Occasionally the clay is found filling isolated and deeper pockets-"pot-holes"-in the gravel, whose origin might be traced to tempo­rary eddies formed in the stream.

Looking now at the probable history of the gravels and the clays, we must bear in mind certain important facts. In the first place, the clay and the red gravel have' been traced for a very considerable distance up the Delaware-the clay, itself, to Trenton and far beyond-and nowhere do they seem to extend very many miles on either side of the river. Similar deposits occur along the Lehigh, and there, too, the clay rises to the 150 foot line. Manifestly enough, streams occupying the present channels of the Delaware and Lehigh, but rising to much higher levels than the modern streams, must have been instrumental in depositing the materials of this drift; this assumption we find borne out by the line of direction which the pebbles, took in reaching their destination. The yellow gravel, on the other hand, is not restricted to any marked out water-course, but extends from the Delaware clear through the southern portion of the State of New Jersey, and far down the Atlantic border of the United States. It appears, therefore, to be of oceanic origin. Putting the two series of facts together, we have presented to us, in the first place, the sub­mergence of the land, and the deposition by the sea of the yellow [PAGE 128] gravel; secondly, the re-elevation of the land, and the formation of the channel of the Delaware; and thirdly, the great Delaware flood, which swept over the existing lowlands, and threw the boulder-bearing clay to a height of 150 and more feet above the present level of the stream. During the period of submergence the oceanic billows broke within the confines of the State of Pennsylvania, and the greater portion of the site of Philadelphia formed part of the dominions of the sea. When, after the land had again been laid dry through elevation, the northern torrents swept resistlessly on to the sea, the city was again deluged by a sea of water, this time fresh, whose surface rose to an altitude corresponding to the pinnacles of many of our tallest church spires.

What the exact age of the yellow gravel is has not yet been definitely determined, but as it rests in New Jersey upon Tertiary deposits it can evidently not be older than these. Possibly it belongs to the closing period of the epoch. Turning back to our clay and red gravel we might naturally inquire, What could have been the nature of the flood which brought them here? Geologists tell us that in by-gone ages, when climatic conditions were probably very different from what they are now, an enormous sheet of ice, or glacier, covered the greater part of the northern United States east of the Mississippi River; that this continuous ice mass, moving southward, heaped in front of it a long line of earth and boulders similar to that which every Alpine glacier heaps in front of it at the present day, and which geologists technically term a "terminal moraine;" and that after a given period, the glacial epoch," the ice began to thaw and eventually completely disappeared. Here, then, in the melting of the ice, we have a possible and even plausible source for our southerly trending torrents. Now, what is the nature [PAGE 129] of the evidence upon which geologists have based their assumption of glaciation? In the first place, we have the long line of boulders, frequently as much as 50-75 feet in height, and from a few hundred feet to a full mile in width, which, rising and falling with every alternation of mountain and valley, stretches in one almost continuous sweep from the Atlantic half across the continent. The materials of this moraine are indiscriminately heaped together, and in no way appear to be sorted into definite layers. Their rounded surfaces in very many cases indicate that they must have traveled, and this is further proved by the circumstance that in most places the character of the moraine rock is very different from the character of the rock upon which it rests; but in order to have traveled something must have pushed them, and this could only have been either water or moving ice. It is easily conceivable that at one time a vast body of water might have extended into the continent of North America, and formed for itself a beach line co-extensive with the line of the moraine; in so far, no valid objection can be made to the theory which views this omnium gatherum of boulders in the light of an ancient beach. But it is not so easy to account for the up and down disposition of the materials, and in fact we know that no beach line could possibly have been formed this wise, creeping up mountain slopes and descending into valleys. We are thus forced to give up the notion which ascribes the building of this barrier to aqueous agencies, and are driven to the alternative of ice action. Whether it was one individual sheet of ice, hundreds or thousands of feet in thickness, traveling over mountain-top and valley, or a more or less close union of several independent streams, which produced the effects before us, it may be a little premature to say; but there are sufficient facts before us to show that ice was [PAGE 130] the moving power. We find, for example, that north of the line of the moraine, the solid rock, where laid bare from an earthy covering, is variously grooved and polished, the result of attrition between its surface and rock particles that must have been moved and hard-pressed over it.This same feature is exhibited all through the Alpine glaciated regions, and can there be directly traced to the moving ice and its imbedded boulders, but it is completely wanting south of the line. And in the materials of the moraine itself we meet with numerous boulders showing parallel grooves or scratches, the result of similar attrition under pressure.Evidently, direct ice action extended as far as the line of the moraine and no further. This line, which on the Atlantic border reaches its most southerly point at Perth Amboy, extends thence westward and crosses the Delaware River at Belvidere, a few miles south of the Water Gap.The ice sheet advanced within about sixty miles of our metropolis, and then halted. Just when this event took place it is impossible to say, but it was, doubtless, a goodly number of thousands of years ago. A time came to pass when the ice melted; torrents of water were given out, which, bursting through the moraine rampart, swept the materials far and wide, and laid the lowlands under a heavy contribution of boulder-debris and mud. These torrents were largely gathered up in the pre-existing water-courses, and such primary streams as the Delaware and Lehigh were swollen to the magnitude indicated by the 150 foot line of clay already several times referred to. Herein we have the origin of the Philadelphia red gravels and brick (or boulder) clay, time-indices of the floods that deluged the country. Large cakes of ice, drifting from their anchorages, doubtless navigated the ice-cold waters, and melting or stranding, deposited the share of rock with which they may have [PAGE 131] been freighted. To such transports, probably, do we owe the presence of the larger boulders that here and there lie buried in the clay.

But it appears that the great ice-sheet did not all melt at once; in time the angry waters abated their fury, and a period of former quiescence again set in. The rivers had withdrawn to their legitimate channels, and left their banks to the mercy of those powerful tools which nature employs in carving the earth's surface. Once more the ice melted, and once more the resulting torrents swept resistlessly in the valley of the Delaware. This final thaw was, however, of far less significance, judged by magnitude alone, than the previous one, and its effects appear correspondingly confined to much narrower limits. The newer glacial, or " Trenton," gravel, which is beautifully exposed in long banks a little north of Trenton in the line of the Pennsylvania Railroad, and which in one place in the vicinity of that city has a development of some fifty feet, acquires no special importance about Philadelphia, rising only about twenty-five feet above mean water line. It lies close on the river front, and on Market Street extends up to about Third Street. It forms the floor of the river opposite the city (and as far north as Trenton), and in certain localities, as on Smith's Island and on the bar opposite Cooper's Point, attains a thickness of 100 feet. In a boring recently conducted on Black's Island, a little below Fort Mifflin, the gravel was found to extend to at least 175 feet. Higher up the river, as at Bristol, the gravel, with its topping of light sand, extends fully two miles inland, and in some places still further. Boulders are very numerous in the upper course, and are trans­ported to Philadelphia and elsewhere as "cobble-stones." Apart from its purely physiographical relations the Trenton gravel acquires [PAGE 132] special importance from the circumstance of its having yielded, as claimed, the earliest record of man's existence on this continent. Rude stone fragments, very like the chipped implements (palseoliths) left by man of the oldest stone age of Europe, have from time to time been reported from the deposits about Trenton, and if fully substantiated in their character place man coeval with the great northern ice-sheet.

Visitors to the region about Chestnut Hill, Bryn Mawr, Haverford, Coopertown, and elsewhere on the high ground, will have noticed, at elevations of from 300 to 450 feet, patches of a rusty-looking pebbly or gravelly deposit, very different from anything that we have thus far seen, and much resembling in places an artificial compound. The individual pebbles of quartz are largely cemented together into a coarse iron conglomerate, which sometimes rings when struck with a blow of the hammer. Very little of this gravel is found in the lowland, and where so occurring is derived as a down-wash from the upper region. Patches of the same material, on the other hand, are met with on the lowland hills of New Jersey, as about Mount Holly, and on the slopes of the Highlands of Sandy Hook, where in both cases they cover strata of Cretaceous age, and are thus proved to be of a newer date. Further south along the Atlantic border the line of gravel, always occupying a high position, can be traced beyond Wilmington to Georgetown Heights, District of Columbia, and deep into the Southern States. It is more than probable that the disjointed patches of this "Bryn Mawr gravel" or "Mount Holly conglomerate," at one time formed a continuous whole, which has gradually been reduced to its present condition through the effects of erosion. The formation is undoubtedly of marine origin, and [PAGE 133] as we have already been taught that a pebbly deposit indicated littoral conditions, we must conclude that it is in the nature of an ancient shore-line, or beach. But of what age? We have already determined it to be post-Cretaceous, and it can, therefore, only be Tertiary or post-Tertiary, more likely the former, seeing the amount of erosion it has suffered. Nearer to the sea the deposits of equivalent age are probably represented in some of the clays and sands that cover the lowland strip.

Such, in brief, is the wonderful history which the gravels, clays, and cobble stones of Philadelphia reveal to us-a history which cannot fail to sharpen the edge of even the most dormant intellect. The mind wanders back to a period, possibly long antecedent to the advent of man, when oceanic conquests extended far within the boundaries of our forest State; when the Atlantic billows swept the crests of our most elevated hills, and when the heights of Chestnut Hill, Bryn Mawr, and Media echoed back the roar of the breaking surf. The beach-line of upland gravel was then deposited. Southern New Jersey, which had already added its mite to the formation of the continent, lay at this time buried beneath the sea, and had again to be; from its submerged surface occasional promi­nences probably rose out of the water, forming islands, some of whose positions may still be indicated in such lowland hills as the Sandy Hook Highlands and Mount Holly. The land now began to rise, but before New Jersey had finally and completely emerged from the deep, the ocean deposited the yellow gravel. The site of Philadelphia was made part of terra firma, and by its side was dug the channel of the Delaware, which had before debouched not far from the city of Trenton. The marine fauna of the coast was then the same as now, and even on the land the animals were very little [PAGE 134] different; a few uncouth forms still wandered among the many, strangers to the flock. In the meantime certain physical conditions, but little understood, had gradually prepared the way for the accumulation in the north of a vast ice sheet, which advanced southward to within about sixty miles of the city, and whose melting produced those prodigious floods whose effects are portrayed in the deposits of the now familiar gravel and clay. The greater part of what is city was buried beneath water of icy-coldness, and icebergs of no mean magnitude traversed the region of the present church-spires. The reindeer and mastodon, precursors of the storm, had wandered with the climate south, and left in their bones the tale of northern winters. Man either had appeared, or was to appear; not, however, the civilized man of to-day, but the feeble-minded savage, who fashioned rude stone implements, and who had possibly only recently acquired the use of articulate language.

The ice finally disappeared, the waters were called back to their channels, and the landscape rose resplendent in its modern garb.

The traveler to the city who sees the gravels swiftly fleeting before the windows of his railway carriage; who laments the singularly unpretentious appearance of the outlying brick-yard clays; and who ruthlessly treads the much, and justly, despised cobble­stones of our thoroughfares, little thinks what history these commonplace objects unfold. They stand as monuments of a history far more wonderful than any written in book, and will continue to so stand probably long after man will himself have disappeared from the scene.



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