One of the large questions geologists grappled with in the early to mid-19th century was to explain certain phenomena that were often found together on the landscape: large boulders of a rock type unrelated to the rocks around it (for example, a granite boulder on a basalt ridge or an expanse of sandstone); piles of sand and scattered rock debris; and scratches (striations) all pointing in the same direction on the surface of bedrock and heavy boulders. Did these associated features arise from the same cause? What could explain them?
In 1840, Louis Agassiz published Études sur les Glaciers (Studies on Glaciers), advocating the idea that there had been an "Ice Age" in which much of northern Europe and North America were covered by a continent-sized ice sheet he termed a “Mer de Glace” (“Sea of Ice”). The publication set off a lengthy controversy among contemporary geologists in the Old and New Worlds. Despite the impressive array of evidence presented by Agassiz in support of the glacial theory—striations left upon bedrock, the presence of large erratic boulders, and extensive deposits of unsorted sand and gravel—the vast majority of his colleagues were unwilling to accept such far-reaching conclusions about the occurrence and actions of ice sheets (large or small) in the recent geological past.
Edward Hitchcock was no stranger to so-called drift phenomena, the same striations, erratic boulders, and rock debris that Agassiz sought to explain. He agreed that the pattern demanded an explanation, but disagreed with Agassiz's solution to the question. Hitchcock's stance both exemplified the skepticism held by most other American geologists and seemingly influenced their thinking into remaining uncommitted towards Agassiz’s, or any other, theory behind these natural phenomena.
On April 5, 1841, Hitchcock addressed the newly founded Association of American Geologists (AAG), of which he was president, at its second annual meeting. A large portion of his talk was devoted to the ‘drift problem’ in America and the close resemblance of these phenomena to the same in Europe. He summarized twelve lines of inductive evidence and expressed skepticism towards “every particular theory” advanced to explain it, including the new glacial theory, though admitting that he had not yet read Agassiz’s volume. However, during the coming summer, two important developments occurred.
The first was that through the aid of Benjamin Silliman, Hitchcock secured an edition of Agassiz’s book and copied out extracts (in French) of its leading ideas, chiefly regarding the evidence of alpine-style (that is, originating in mountains) glaciation. Now better informed, by the time he published the text of his address later that same year, he claimed that he “seemed to be acquiring a new geological sense; and I look upon . . . the tout ensemble [sum total] of diluvial phenomena, with new eyes.” While stressing the theory’s positive aspects, Hitchcock went so far as to state that, “so remarkably does it solve most of the phenomena of diluvial action, that I am constrained to believe its fundamental principles to be founded in truth."
The second important development was an observational test for evaluating the validity of alpine-style glaciation that Hitchcock conceived and undertook that summer in an excursion to the White Mountains of New Hampshire, accompanied by his wife, Orra. He selected Mount Washington for his assessment, knowing it as the highest peak in the eastern United States, with an elevation of over 6,000 feet.
It was well known among geologists that the principal direction in which the drift phenomena were displayed throughout most of New England was towards the southeast, regardless of the elevation at which they were observed. Taking what he had learned from Agassiz, Hitchcock formulated a hypothetico-deductive argument. Assuming that the White Mountains would likely carry the marks of any previous glaciers, he wrote, “these mountains . . . would form a centre, or an axis of dispersion, from which glaciers would have carried bowlders [sic] outwards, like the Alps in Switzerland” (i.e., in many different directions). “On the other hand,” he added, if evidence could be found “on the summit of these mountains that glacio-aqueous agency took a southeasterly course there, it would render it extremely probable that no such centers of dispersion exist in the United States; and that no true glaciers have descended from our mountains.” Hitchcock’s fieldwork on Mount Washington, up to an elevation of some five thousand feet, amassed evidence that all of the drift phenomena noted there were in accordance with the general southeasterly direction seen in features at lower altitudes. Thus, he became convinced that alpine-style glaciers could not have been responsible for the drift.
While Hitchcock’s reasoning was sound, it also seemingly closed his mind to further consideration of Agassiz’s more radical hypothesis of a former continent-sized ice sheet. If no alpine-style glaciers had descended the slopes of Mount Washington, then what was the likelihood that a vast Mer de Glace had produced them in the first place? What he and other geologists were unable to imagine was that such an ice sheet did not require a high mountainous center to be formed. Only one (very remote) possibility remained open to Hitchcock’s thinking; namely, “that such a point, or axis of dispersion, if it exist[s], must lie very far to the north.” But whereas “[n]o such lofty mountains, I believe, have yet been discovered in the northern part of this continent[,] [t]hey must certainly lie north of Hudson’s Bay, if they exist.” Ironically, Hitchcock’s suggestion of a possible center of dispersion, in the region of Hudson’s Bay, was vindicated by a later generation of geologists, but without assuming a major mountain range to act as a catalyst for the growth and radiation of a continental ice sheet.
At the time, it appears that Hitchcock’s claim for the lack of geological evidence regarding a center of dispersion within the White Mountains formed one of the strongest arguments against Agassiz’s glacial theory. Two years later, American geologist Henry Darwin Rogers again drew attention to Hitchcock’s conclusions (seemingly confirmed by a similar lack of evidence gathered from the Adirondack Mountains by New York geologist Ebenezer Emmons) “that those high tracts have not been centres of dispersion, but like every other spot, have been invaded by the drift from the north.” Thus, Hitchcock’s abrupt retreat from the glacial theory returned him to his prior, skeptical stance regarding any theory of the drift phenomena. The nearest that he would come was to ponder whether “the ultimate and true theory on the subject may be compounded of them all?”, namely, some combination of ice and water that he had previously labeled “glacio-aqueous action.”
Even Hitchcock’s subsequent analysis of the Richmond boulder trains, discovered in northwestern Massachusetts, did not change his mind regarding theories of the drift. He confessed to being unable to explain their patterns of dispersal by “agency of currents of water alone,” nor by “floating icebergs.” “But when we come to examine the country with reference to a glacier,” he wrote, “we shall find it about as difficult to imagine the existence of one there as of a river.” Hitchcock merely declared that the phenomenon in question would have to remain “unexplained till more analogous facts shall have been observed.”