Frozen Earth: The Once and Future Story of Ice Ages by Doug Macdougall Page B

quite efficient at sorting out the chaotic jumble of particles according to size and weight.It winnows the unsorted glacial drift, dumping the largest pieces at the base of the glacier or close to its boundary, and carrying the smallest grains in suspension over long distances.The meltwater streams build up sand bars in some places, gravel bars in others, and when they are flowing at full force, they sometimes carry quite large boulders along with them.In the northern United States and in Canada, in Scandinavia and northern Europe, man has taken advantage of this combined production and sorting process that is a relic of past glacial action.The sand and gravel deposits of the meltwater streams are scooped up by the truckload and used as construction materials.In overall economic importance, these deposits overshadow all other kinds of mining activity.In these same regions, the action of ice and meltwater has left an aesthetic legacy in addition to a practical one: the undulating topography (and the sand traps) of many a well-groomed golf course.
    The very finest particles of rock carried by the meltwater streams, much smaller than sand grains, are sometimes called rock flour.They are produced by the scouring action of the ice on underlying bedrock, and they are so fine that they remain suspended for very long periods of time and give glacial lakes their characteristic brilliant blue-green color.The scraping and scratching and polishing that produces rock flour leaves very distinctive telltale marks on the underlying bedrock.But it is not the ice itself that does the grinding; even hard, brittle ice at temperatures well below freezing cannot gouge out scratches and grooves in solid rock—it is simply not hard enough.Yet Agassiz and other careful observers of the Swiss glaciers found such features on rocks near the edges of glaciers.They noticed that the scratches were most prominent in areas where the ice had recently retreated, but, like the erratics and drift, could also be found far from any contemporary glaciers.They soon realized that it was actually the rock debris carried by the ice that was producing the scratches.Rocks and pebbles embedded in the ice were being dragged across the underlying surface; the glaciers were like gigantic sheets of sandpaper smoothing out the rocks beneath.In the process they produced the rock equivalent of sawdust: glacial rock flour.When scientists were eventually able to map out the movement of ice within glaciers, they discovered that the base of a glacier is continually being renewed with ice from above, complete with its embedded grit.The natural sandpaper is constantly being refreshed.
    In most places that were glaciated during the Pleistocene Ice Age, scratches and grooves and polished rock surfaces are very abundant.Once you know what to look for, they seem to pop up everywhere.Recently, I walked along the wonderful meandering stone wall built by the artist Andy Goldsworthy at Storm King, an art park not far from New York City.The wall was made from local stones, with those in the top layer chosen for their flat surfaces.It was the Pleistocene glaciers that left them with these surfaces—most show the characteristic scores and scratches of glacial scouring.They are a reminder that just twenty thousand years ago the region was thick with ice.

    Figure 7.A cartoon sketch of Professor William Buckland by the mining engineer Thomas Sopwith titled “Costume of the Glaciers,” showing Buckland dressed for fieldwork.The numerous captions are difficult to read, even in the original, but the lines at Buckland’s feet are noted to be “Prodigious Glacial Scratches” produced by “the motions of an IMMENSE BODY .”Other captions are summarized in the text.From Mrs.E.O.Gordon, The Life and Correspondence of William Buckland (London: John Murray, 1894).

    In Britain, when the debate over the ice age theory was raging in the middle of the nineteenth century, a well-known mining engineer