Boulder clay

“ There are no signs of grinding or thrusting of their surface, such as must have been apparent had the clay been formed beneath a covering of ice sliding over the land. This latter mode of formation is accepted by many as a true explanation of the phenomena presented by the Boulder-clay; but for this reason especially I differ from that conclusion. Moreover, if the clay be a direct result of ice moving over the land, an emergence after the deposition of the Middle Glacial gravels must have intervened ”

“ The stability of such soil is governed not only by the nature of the clay, but by that of the boulders and their effect upon the earth in which they are embedded, and much depends upon the degree of changeableness upon exposure to air and moisture of all the particles of which they are composed; hence boulder-clay, although hard to excavate, may quickly dissolve. On the contrary, it may occasionally be so hard that it seems to be solid rock, and may even resist erosion and weathering as well as if it were rock ”

“ By careful washing, most boulder clays may be purified sufficiently to enable coarse brown pottery to be made from them. Clean deposits of sufficient size to be worked without any purification are occasionally found. [67] Usually, however, the boulder clay formation is somewhat treacherous as it is difficult to ascertain its nature; boreholes are apt to be quite misleading as the formation is so irregular in character. ”

“ The gravels are made up of exactly such material as would be derived from the waste of Boulder-clay—rolled lumps of hard and soft chalk, flint pebbles, with some fragments of quartz, limestone, and other derived rocks, enclosing broken pieces of derived fossils. They constitute also a gradually descending series, without any very great break in their continuity, either in regard to level or position. All the patches appear to occupy hollows in the surface of the clay, by which circumstance they have, indeed, been preserved—that is, down to the level of the Preglacial valley-system ”

“ The clay or brick-earth is of a reddish hue, and rests upon a chalky Boulder Clay, which is exposed farther up the hill. It ranges in thickness from about 4 to 6 feet; and above it are sands and gravel, the latter varying in thickness from about 2 to 6 feet, and of much the same character as that of the Warren Hill pits, but containing far less chalk. The sand occasionally comes down in pipes or pockets into the clay, and some of the worked flints occur in it, as well as in the clay. ”

“ It is probable that the gravels extend beneath the clay over nearly all the intervening area, in greater force along the lines of the larger valleys (which I believe to have been carved into nearly their present form in preglacial times) , and in a more attenuated, or even patchy condition under the higher grounds.
But the gravels can in no instance of which I am aware be traced up to the escarpment of the Chalk, or, in other words, beyond a certain definite level. It is not that they disappear beneath a great thickness of Boulder-clay to reappear at its opposite boundary ”

“ But courage, patient hearts! the boulder-clay will one day yield up its secret too. Still further on by a few hundred yards, I could have again found use for the calotype, in transferring to paper the likeness of a protuberant picturesque cliff, which, like the Giants' Graves, could have belonged, of all our Scotch deposits, to only the boulder-clay. It stands out, on the steep acclivity of a furze-covered bank, abrupt as a precipice of solid rock, and yet seamed by the rain into numerous divergent channels, with pyramidal peaks between ”

“ But, if we assign to these hoar relics of long-vanished generations of men the greatest age that can possibly be claimed for them, they are not older than the drift, or boulder clay, which, in comparison with the chalk, is but a very juvenile deposit. You need go no further than your own seaboard for evidence of this fact. At one of the most charming spots on the coast of Norfolk, Cromer, you will see the boulder clay forming a vast mass, which lies upon the chalk, and must consequently have come into existence after it. ”

“ Fragments of still more modern rocks occur in the boulder-clays of Caithness. My friend Mr. Robert Dick, of Thurso, to whose persevering [Pg 290] labors and interesting discoveries in the Old Red Sandstone of his locality I have had frequent occasion to refer, has detected in a blue boulder-clay, scooped into precipitous banks by the river Thorsa, fragments both of chalk-flints and a characteristic conglomerate of the Oölite. He has, besides, found it mottled from top to bottom, a full hundred feet over the sea-level, and about two miles inland, with comminuted fragments of existing shells. ”

“ The next deposit, supposing these patches of sand and gravel to be of one age—a very doubtful matter,—is the Boulder Clay—a stiff, tenacious clay, often studded with pieces of chalk, from minute grains to biggish lumps, which commonly are fairly well rounded, together with flints, both rounded and angular, and fragments of other rocks. ”

“ Boulder clays, when freed from stones and gravel, are sufficiently plastic to meet the needs of most users, without being so highly plastic and contractile as to necessitate admixture with sand or similar material. ”

“ These great ice-streams created the geological formation called “The Drift,” or boulder-clay, which we have at Woodhall. The clay is simply the detritus, produced by the grinding, through long ages, of the rocks under the vast and weighty ice-fields slowly moving over them, and the abrasion of the hill-sides which they scraped in their course. ”

“ Agriculturally, drift or boulder clays are poor soils, but by judicious management and careful mixing they may be made more fertile. Where it contains chalk—as in Norfolk and Suffolk—boulder drift forms an excellent arable soil. ”

“ It is not all stones. Clay and earth mingle with it, often enclosing the stones; and the debris left by extinct glaciers of ages ago is sometimes called the boulder-clay. This is the deposit, earthy and stony, that the glacier leaves on the floor of the valley as it shrinks—unless the river which usually springs from the end of glaciers sweeps it away. Most of the stones thus left are smoothed or polished and covered with scratches or ruts, such as would be made by rubbing against other hard pointed fragments of stone. ”

“ It is much the same as that of the Thames; but as we are in a glaciated area we find, as in the Fenland, that much of the erosion had taken place before or during the Glacial Epoch, for boulder clay occupies part of the valley.
Fig. 3.
Boulder clay or till not only occupies part of the valley, it descends far below the present river bottom and even below the lowest submerged forest. This we find always to be the case in the glaciated parts of Britain ”

“ They consist of a boulder clay in the N. and of loess in the S. The former presents an intimate mixture of boulders brought from Finland and Olonets (with an addition of local boulders) with small gravel, coarse sand and the finest glacial mud,—the whole bearing no trace of ever having been washed up and sorted by water in motion, except in subordinate layers of glacial sand and gravel; the size of the boulders decreases on the whole from N. to S., and the boulder clay, especially in N. and central Russia, often takes the shape of ridges parallel to the direction of the motion of the boulders. ”

“ However it may have originated, I cannot believe with Professor Hughes that, in the Pont-newydd cave, it was washed in through a swallow-hole, from the Boulder- clay of the neighbourhood, by a freshwater stream. The relative proportion of stones of different kinds is not nearly the same in the pebble-bed and in the Boulder-clay covering the surface of the ground at a higher level. In the pebble-bed nearly all the stones, so far as I could see, are Denbighshire sandstone or grit. In the Boulder-clay a large proportion of the pebbles are limestone. ”

“ In other localities, currents brought in a continual supply of sand, or washed off the boulder mud and sand, and then re-deposited them in irregular beds; hence resulted those local deposits of stratified sand and gravel so frequently to be seen resting over the boulder clay. At length, by degrees, the land emerged from the sea, yet glaciers still capped its hills and choked its valleys; but eventually a warmer and more genial climate arose, plants and animals, such as those at present amongst us, and some, such as the wolf, no longer extant, were ere long introduced ”

“ We see from this letter that the thought of the mystery of the great boulder haunted Darwin for years, but the young geologists of the Survey were confronted not with one boulder, but, day after day, week after week, with an accumulation of only half-explained mysteries. When they started work the view that a large part of the British Islands had been covered by land ice, and that the boulder-clay was the record of its passage, had, after a period of neglect, again come into prominence; but it was [Pg 28] very far from being universally accepted (see the historical discussion in Part II.) . ”

“ To some extent, and in small areas, this may be so; but looking at the Boulder-clay as a whole, there is a striking regularity in its occurrence. When viewed on a true scale, in the exceptional case of the Chalk escarpment it is seen to plunge down about 500 feet; but this being in a distance of not less than 10 miles, represents a fall of 1 in 100 only, or an angle of about half a degree. It touches, as any other formation would be expected to do, the outliers and hills within the larger valley which are sufficiently high to reach its horizon ”

“ Along this line, especially in the vicinity of Hill lake, there is a zone of deep clays of very promising agricultural value. In the area of granite farther north, the whole future, so far as agriculture or forestry is concerned, depends upon the general depth of the boulder clay. From the mere fact that it is a drift deposit, this depth constantly varies, but only a detailed soil survey of each township could show where and how much. There are large areas of almost pure rock outcrop and muskeg of little or no value even for timber production. ”

“ We have already seen that the changes in plant and animal life point to a change from a hot climate, through a temperate, at last to arctic cold. Again, over the greater part of Northern England the rocks of the various geological periods are buried under sheets of tough clay, called boulder clay, for it is studded with boulders large and small, like raisins in a plum pudding. No flowing water forms such a deposit, but it is found to be just like the mass of clay with stones under the great glaciers and ice sheets of arctic regions ”

“ Near to Durham, boulder clay, containing large polished and striated blocks, overlies stratified sands, which are about 30 feet in thickness. Excepting at Tynemouth, where Mr. Howse found small pieces of Cyprina Islandica, a marine shell still living on our coast, I know of no fossils which have been discovered within our district belonging to the boulder clay era.
So far as is at present known, moving ice is the only natural agent which produces such dressed surfaces; and therefore it may be inferred, that during the boulder clay era, the North of England had an arctic climate. ”

“ It is reasonable to conclude that the till or boulder clay, under the Leda clay, belongs to the earliest period of probably gradual subsidence, accompanied with a severe climate, and with snow and glaciers on all the higher grounds, sending glaciated stones into the sea. This deduction agrees with the marine shells, polyzoa, and cirripedes found in the boulder deposits on the lower St. ”

“ It must contain, in heterogeneous admixture, the great boulders, the lesser rock fragments, the gravel chips, the sand, and the slimy mud; these settling down quietly in the cold, gloomy waters, overshadowed by the great ice-sheet, must form just such an agglomeration as we find in the boulder clay and tills, and lie just in those places where these deposits abound, provided the relative level of land and sea during the glacial epoch were suitable. ”

“ The Chalk comprises all the main divisions of the formation, including the Totternhoe stone, Melbourn rock and Chalk rock. Much glacial boulder clay covers all the higher ground of the county; it is a stiff brownish clay with many chalk fragments of travelled rocks. Near Ely there is a remarkable mass of chalk, evidently transported by ice, resting on and surrounded by boulder clay. Plateau gravel caps some of the chalk hills, and old river gravels occur at lower levels with the bones of mammoth, rhinoceros and other extinct mammals. ”

“ About twelve years ago, a large erratic block, above four feet in diameter, was dug out of the boulder clay at Icklingham, which I found to consist of a hard siliceous schist, apparently a Silurian rock, which must have come from a remote region. The tool-bearing gravel here, as in the case to which it has been compared near Bedford, is proved to be newer than the glacial drift, by containing pebbles of basalt and other rocks derived from that formation. ”

“ There is a widespread covering of boulder clay especially in the northern part bordering the shore, where it contains fragments of shells and includes numerous boulders which have been carried eastwards from the high grounds west of the Moray Firth. ”