Boulder-clay is a stony clay which has a very wide geographical distribution. It occurs in the British Islands, as far south as the Bristol Channel and the valley of the Thames. It overspreads extensive areas in Scandinavia, Holland, Northern Germany, and Central and Northern Russia. In the lower valleys of the Alps, and many other mountain districts of Central and Southern Europe it is also well known. It has likewise been traced over vast regions in British America and the northern states of the Union. Boulder-clay varies in thickness from a few feet up to 20 or 30 yards—being generally thickest upon low-lying regions, and thinning away as it is followed up towards the mountains. It is unstratified, and contains stones scattered confusedly through its mass without any reference to their relative size and weight. These stones are of all shapes and dimensions, from mere grit up to great blocks several tons in weight. Many of them are blunted and subangular, and show on one or more sides that smoothed, abraded, and striated appearance which is characteristic of glacial action. The clay itself is generally more or less tough and hard, and has apparently been subjected to intense pressure, as is further shown by the presence now and again of a kind of 'pseudo-lamination' or 'pseudo-bedding,' often marked by differences of colour, and sometimes by lines of stones. These structures appear to be due to the shearing and yielding of the clay under pressure. Here and there lenticular patches of water-worn stones, and gravel, sand, and clay occur completely inclosed in the tumultuous stony clay—being confused, contorted, and involved with the latter in such a way as to show that they and the stony clay had been rolled forward upon each other. The included stones are more or less local in character—that is to say, the great majority in any individual sheet of boulder-clay have been detached from rocks belonging to the drainage area within which that particular sheet of boulder-clay occurs; while those fragments which have come from a greater distance can be shown to have travelled in one and the same direction as the locally derived stones and boulders. In like manner, the colour of the clay is more or less local. Thus, in regions composed largely of red-coloured rocks, the boulder-clay is red; while it may be yellow, gray, or blue in other regions, according to the prevailing colours of the rocks upon or near to which it lies. Fossils have been met with at rare intervals in boulder-clay, but these are always derivative, and not unfrequently they are smoothed and striated in the same manner as the stones; in other words, they are boulders just as are the stones amongst which they lie. In many places, where the boulder-clay has been stripped off the underlying rocks, these are found to present a highly smoothed and abraded surface, streaked with rectilinear grooves and scratches, the trend or direction of which corresponds with that followed by the stones and boulders in the boulder-clay. Not unfrequently, however, the underlying rocks present a very shattered and confused appearance under the clay—larger and smaller masses being inclosed in the lower part of the boulder-clay—the bottom portion of which, indeed, is often entirely composed of the debris of underlying rock-masses, which have all the appearance of having been torn out of their places and dragged forward in the direction followed by the further-travelled blocks and boulders of the boulder-clay. In the lower reaches of mountain valleys, as in those of Britain, the Alps, the Pyrenees, &c., boulder-clay tends to form gently undulating or approximately level sheets or layers, which being cut through by streams or rivers, form more or less bold bluffs and scours. In some low-lying tracts, however, it is often arranged in long parallel banks or 'drums,' and 'sowbacks'—the general trend of which corresponds with that of the striæ or scratches on the underlying rocks, and with the direction followed by the boulders in the boulder-clay. Excellent examples of drums occur in Nithsdale and in the lower valleys of the Teviot and the Tweed.
In these and many other regions the boulder-clay is frequently found heaped up on one side of prominent rocky knolls and hills, the steep faces of which front the direction in which the boulders of the clay have travelled. This is the appearance called 'crag and tail'—the tail being composed largely of boulder-clay. As examples may be cited Edinburgh Castle, Dalmahoy Crag, and other isolated hills near Edinburgh. The origin of boulder-clay, which has been the subject of much discussion, is now hardly in dispute. Boulder-clay is unquestionably the product of glaciation, and the phenomena mentioned above lead to the belief that this stony clay is simply the bottom-moraine or ground-moraine of extinct glaciers, which formerly had a most extensive development in the northern and temperate latitudes of the globe (see GLACIAL PERIOD, KAIMES, MORAINES). Boulder-clay must not be confounded with the marine clays, which contain the remains of marine shells, &c. of arctic and northern species. These clays are usually more or less well bedded, and the stones contained by them have evidently been dropped from floating ice. Boulder-clay is known in Scotland as Till—a term which many geologists prefer, inasmuch as boulder-clay is often rather a stony earth than a clay. Formerly the terms Diluviu and Drift were used to designate boulder-clay and certain gravelly and arenaceous deposits frequently associated with that accumulation; but they are now seldom or never used. In the Alpine regions boulder-clay is known under the names of Grund-moraine, Moraine profonde, or Moraine de fond; in Germany it is called Geschiebemergel, Geschiebeleh, or Blockleh; and Krosstenslera in Sweden.