rpentier told the story of how in 1834 he was walking along a countrylane with a swiss woodcutter when they got to talking about the rocks along the roadside。 thewoodcutter matter…of…factly told him that the boulders had e from the grimsel； a zone ofgranite some distance away。 “when i asked him how he thought that these stones had reachedtheir location； he answered without hesitation： ‘the grimsel glacier transported them on bothsides of the valley； because that glacier extended in the past as far as the town of bern。’ ”
charpentier was delighted。 he had e to such a view himself； but when he raised thenotion at scientific gatherings； it was dismissed。 one of charpentier’s closest friends wasanother swiss naturalist； louis agassiz； who after some initial skepticism came to embrace；and eventually all but appropriate； the theory。
agassiz had studied under cuvier in paris and now held the post of professor of naturalhistory at the college of neuchatel in switzerland。 another friend of agassiz’s； a botanistnamed karl schimper； was actually the first to coin the term ice age （in german eiszeit ）； in1837； and to propose that there was good evidence to show that ice had once lain heavilyacross not just the swiss alps； but over much of europe； asia； and north america。 it was aradical notion。 he lent agassiz his notes—then came very much to regret it as agassizincreasingly got the credit for what schimper felt； with some legitimacy； was his theory。
charpentier likewise ended up a bitter enemy of his old friend。 alexander von humboldt； yetanother friend； may have had agassiz at least partly in mind when he observed that there arethree stages in scientific discovery： first； people deny that it is true； then they deny that it isimportant； finally they credit the wrong person。
at all events； agassiz made the field his own。 in his quest to understand the dynamics ofglaciation； he went everywhere—deep into dangerous crevasses and up to the summits of thecraggiest alpine peaks； often apparently unaware that he and his team were the first to climbthem。 nearly everywhere agassiz encountered an unyielding reluctance to accept his theories。
humboldt urged him to return to his area of real expertise； fossil fish； and give up this madobsession with ice； but agassiz was a man possessed by an idea。
agassiz’s theory found even less support in britain； where most naturalists had never seena glacier and often couldn’t grasp the crushing forces that ice in bulk exerts。 “could scratches and polish just be due to ice ？” asked roderick murchison in a mocking tone at one meeting；evidently imagining the rocks as covered in a kind of light and glassy rime。 to his dying day；he expressed the frankest incredulity at those “ice…mad” geologists who believed that glacierscould account for so much。 william hopkins； a cambridge professor and leading member ofthe geological society； endorsed this view； arguing that the notion that ice could transportboulders presented “such obvious mechanical absurdities” as to make it unworthy of thesociety’s attention。
undaunted； agassiz traveled tirelessly to promote his theory。 in 1840 he read a paper to ameeting of the british association for the advancement of science in glasgow at which hewas openly criticized by the great charles lyell。 the following year the geological society ofedinburgh passed a resolution conceding that there might be some general merit in the theorybut that certainly none of it applied to scotland。
lyell did eventually e round。 his moment of epiphany came when he realized that amoraine； or line of rocks； near his family estate in scotland； which he had passed hundreds oftimes； could only be understood if one accepted that a glacier had dropped them there。 buthaving bee converted； lyell then lost his nerve and backed off from public support of theice age idea。 it was a frustrating time for agassiz。 his marriage was breaking up； schimperwas hotly accusing him of the theft of his ideas； charpentier wouldn’t speak to him； and thegreatest living geologist offered support of only the most tepid and vacillating kind。
in 1846； agassiz traveled to america to give a series of lectures and there at last found theesteem he craved。 harvard gave him a professorship and built him a first…rate museum； themuseum of parative zoology。 doubtless it helped that he had settled in new england；where the long winters encouraged a certain sympathy for the idea of interminable periods ofcold。 it also helped that six years after his arrival the first scientific expedition to greenlandreported that nearly the whole of that semicontinent was covered in an ice sheet just like theancient one imagined in agassiz’s theory。 at long last； his ideas began to find a realfollowing。 the one central defect of agassiz’s theory was that his ice ages had no cause。 butassistance was about to e from an unlikely quarter。
in the 1860s； journals and other learned publications in britain began to receive papers onhydrostatics； electricity； and other scientific subjects from a james croll of anderson’suniversity in glasgow。 one of the papers； on how variations in earth’s orbit might haveprecipitated ice ages； was published in the philosophical magazine in 1864 and wasrecognized at once as a work of the highest standard。 so there was some surprise； and perhapsjust a touch of embarrassment； when it turned out that croll was not an academic at theuniversity； but a janitor。
born in 1821； croll grew up poor； and his formal education lasted only to the age ofthirteen。 he worked at a variety of jobs—as a carpenter； insurance salesman； keeper of atemperance hotel—before taking a position as a janitor at anderson’s （now the university ofstrathclyde） in glasgow。 by somehow inducing his brother to do much of his work； he wasable to pass many quiet evenings in the university library teaching himself physics；mechanics； astronomy； hydrostatics； and the other fashionable sciences of the day； andgradually began to produce a string of papers； with a particular emphasis on the motions ofearth and their effect on climate。
croll was the first to suggest that cyclical changes in the shape of earth’s orbit； fromelliptical （which is to say slightly oval） to nearly circular to elliptical again； might explain the onset and retreat of ice ages。 no one had ever thought before to consider an astronomicalexplanation for variations in earth’s weather。 thanks almost entirely to croll’s persuasivetheory； people in britain began to bee more responsive to the notion that at some formertime parts of the earth had been in the grip of ice。 when his ingenuity and aptitude wererecognized； croll was given a job at the geological survey of scotland and widely honored：
he was made a fellow of the royal society in london and of the new york academy ofscience and given an honorary degree from the university of st。 andrews； among much else。
unfortunately； just as agassiz’s theory was at last beginning to find converts in europe； hewas busy taking it into ever more exotic territory in america。 he began to find evidence forglaciers practically everywhere he looked； including near the equator。 eventually he becameconvinced that ice had once covered the whole earth； extinguishing all life； which god hadthen re…created。 none of the evidence agassiz cited supported such a view。 nonetheless； inhis adopted country his stature grew and grew until he was regarded as only slightly below adeity。 when he died in 1873 harvard felt it necessary to appoint three professors to take hisplace。
yet； as sometimes happens； his theories fell swiftly out of fashion。 less than a decade afterhis death his successor in the chair of geology at harvard wrote that the “so…called glacialepoch 。 。 。 so popular a few years ago among glacial geologists may now be rejected withouthesitation。”
part of the problem was that croll’s putations suggested that the most recent ice ageoccurred eighty thousand years ago； whereas the geological evidence increasingly indicatedthat earth had undergone some sort of dramatic perturbation much more recently than that。
without a plausible explanation for what might have provoked an ice age； the whole theoryfell into abeyance。 there it might have remained for some time except that in the early 1900sa serbian academic named milutin milankovitch； who had no background in celestial motionsat all—he was a mechanical engineer by training—developed an unexpected interest in thematter。 milankovitch realized that the problem with croll’s theory was not that it wasincorrect but that it was too simple。
as earth moves through space； it is subject not just to variations in the length and shape ofits orbit； but also to rhythmic shifts in its angle of orientation to the sun—its tilt and pitch andwobble—all affecting the length and intensity of sunlight falling on any patch of land。 inparticular it is subject to three changes in position； known formally as its obliquity；precession； and eccentricity； over long periods of time。 milankovitch wondered if there mightbe a relationship between these plex cycles and the ings and goings of ice ages。 thedifficulty was that the cycles were of widely different lengths—of approximately 20；000；40；000； a