Theoretical Part

J. Smith
1900 Transactions of the Geological Society of Glasgow  
The remarkable feature about the shells is that they are nearly all bivalves, as amongst hundreds of specimens collected I have only got some half a dozen univalves, two being Littorina littorea much worn but not scratched, and these, evidently brought from a shore-line, occurring as pebbles in the Boulder-clay. The reason why the shells are, generally speaking, so scarce, is that they had, apparently, a poor time of it, from the immense number of stones which were (if not continually, then at
more » ... ntinually, then at certain seasons of the year) showered down amongst them. At parts where the Boulder-clay is most free from stones the shells are there most common-in such an exposure some scores having been got in a cubic yard of clay. THEORETICAL PART. Formation of the Drift-beds. How were these extensive beds of clay and stones, laminated clay, mud, sand and gravel, boulder-sand, and boulder-gravel, now found in Ayrshire, deposited ? The answer to this question I have, I daresay, anticipated when describing some of the beds; for to one in the field, after the study of a number of the sections, it becomes quite apparent that in no other place than in the sea could they have been deposited. Neither " waves of translation," nor " uphill carnage of blocks and shells," as held by a few geologists at the present day, could account for them. One of the best arguments against Library on May 11, 2015 at University of Manchester http://trngl.lyellcollection.org/ Downloaded from J. SMITH-DRIFT OR GLACIAL DEPOSITS OF AYRSHIRE. 85 these theories is the condition of the shells themselves. Had they been transported by any such agency, how could they have escaped being rubbed? How could many of them have been broken to fragments without having the angles blunted or without even the epidermis being removed? A "wave of translation" would have given us something like the following section :-Fine Mud. Sand. Gravel. Shingle. "Rabbling."* Light Boulders. Heavy Boulders. For it is evident that in greatly agitated waters the boulders would first settle down, followed in order by the other materials (as in this theoretical arrangement) as the waters gradually quieted down, though they may not always and altogether have settled down in one place. But an arrangement like the above is nowhere to be seen in Ayrshire. At the present time in some parts of the Firth of Clyde there are deposits of fine mud going on. Suppose we introduce to the estuary, from glaciers, rivers, and shore-lines, floating-ice carrying sand, mud, and stones, the result would be the deposition of a Boulder-clay, not differing in any particular from the existing shelly pebbly-clay-and Boulder-clay-beds of the drift-formation; that is to say, so far as these beds still remain undisturbed. The boulders in the clays are quite an exceptional feature of the deposits, and it is only from their conspicuous size that portions of the drift-beds have been called Boulder-clays, there fore " stony clays " would really be the proper name for them. Icebergs in the waters of the drift period appear to have been the exception, not the rule. Rotted Rock. That the surface received a great amount of ice-scraping before the deposition of the drift-beds began, there can be no doubt, I * Stones from 5 to 6 inches in size. Library on May 11, 2015 at University of Manchester http://trngl.lyellcollection.org/ Downloaded from 86 TRANSACTIONS-GEOLOGICAL SOCIETY OF GLASGOW. think, for the simple reason that wherever 8 or 10 feet or more of Boulder-clay is removed, as in quarrying, pit-sinking, or railway construction, the underlying rock is seen to be, as a rule, perfectly solid. Now in America and other countries outside the glacial areas, the rocks, even granite, are so rotted from long periods of exposure, that they can be dug into with a spade to a depth of 60 feet. We have, of course, no idea what depth of rotted rock there may have been in this country before the glacial period, but there is every reason to believe it must have been considerable, and that it was all removed before the deposition of the drift began. This may have been done in many places by the first glaciation-ice, and by the sea-waves and by the chafing of shore-ice as the land sank, and much of the striation on rocks may have been produced either by icebergs during the submergence, or afterwards by land-ice. During the glacial period the production of fine mud from the previously decomposed rocks must have been much more simple work for both shore-and glacier-ice than if these agents had had to deal with the rocks in their original solid form. Original Ground Moraine. In Ayrshire there is, properly speaking, I believe, only one ground moraine-or what I have called original ground morainealways thin, and never reaching beyond an inch or two in thickness (see pars. 80 and 125). In cases where Boulder-clay has been dragged by land-ice, by the fronts of glaciers ploughing into the sea-bottom, or by the stranding and grating of icebergs, it has been converted into what may be called secondary ground moraine, and is principally identified by the contained shells beingscratched and polished, by the stones being more intensely striated than those in undisturbed Boulder-clay, and by the interstratified beds being contorted and deformed. The Boulder-clay not a Ground Moraine. That the Boulder-clay is not a ground moraine is clearly seen at several places in the county, as at Gourock Burn (par. 22), and at Blackshaw (par. 115) where red Boulder-clay rests on white rock without being in the least influenced by it even to the extent Library on May 11, 2015 at University of Manchester http://trngl.lyellcollection.org/ Downloaded from J. SMITH-DRIFT OR GLACIAL DEPOSITS OF AYRSHIRE. 87 of the eighth of an inch. This is also well seen where the Boulder-clay rests on the original ground moraine, which is entirely different from it. Boulder-clays are often seen to merge insensibly into laminated clays (par. 174). Deposition of the Boulder-clay. It is a fact worth bearing in mind that the great bulk of the stones in the clay are small-large blocks being exceptional-so that they could have been carried by comparatively thin ice. From rivers and the sea-shore, and from the homely action of a piece of flat wood on the surface of a carried pail of water to keep it from "jauping" over, it may be deduced that the Boulderclay in many cases began to be deposited in comparatively shallow water, the floating icy covering, like the board on the pail, keeping down wave-action, as Dr. David Robertson has pointed out (see Transactions, vol. vii., page 5), so that the mud was not kept in a state of suspension, and thus carried off and deposited in deeper water. The greater part of the boulders, stones, sand, and mud, would be fixed into the bottoms of the glaciers and the shore-ice, so that whenever they floated off deposition would begin to take place. Such a process would keep the Boulder-clay, just as we find it, close to the area from which the material was derived; and as mud sinks rapidly in salt w r ater, as shown by Dr. David Robertson (see Transactions, vol. iv., page 257), it would settle down quickly amongst the stones and boulders. Deposition of Boulder-clay, Sand, Gravel, &c. As the land sank the glaciers would float off as icebergs, depositing, as they melted, stones, sand, and clay on the top of the striated rock-surfaces. Where there were strong bottom-currents the mud would be floated off and deposits of sand and gravel would be left behind (pars. 109 and 158), but where there was still water the clay and stones would accumulate as a massive deposit, occasionally with a few lines of stratification and sometimes with layers of mud (pars. 34 and 156). The absence of marine organisms from much of the drift may be owing to it having been accumulated in darkness under the ice before the latter finally Library on May 11, 2015 at University of Manchester http://trngl.lyellcollection.org/ Downloaded from * " The Great Ice Age," 2nd ed., pp. 72 and 609. Library on May 11, 2015 at University of Manchester http://trngl.lyellcollection.org/ Downloaded from
doi:10.1144/transglas.11.special.84 fatcat:darqkxskd5bixndx722m6yvvr4