BOULDER-CLAY, DILU'VIUM, 'burr, or TILL, is a post-pliocene bed of a remarkable character, and as yet somewhat mysterious history. It usually occurs as the lowest or first of that group of beds which geologists recognize as the post-tertiary, post-pliocene, pleistocene, or superficial formation. The only exception is vhieu a bed of sand inter venes—as is rarely the case—over the surface of the subjacent rocks. It consists of a compact clay, blue or red, according to the prevalent character of the subjacent rocks, having boulders diffused throughout its mass, and with here and there thin lenticular beds of gravel and sand interspersed. Iu some places in Scotland it is not less than 70 ft. thick. In America, it extends to about the 38th parallel; in Britain, it terminates a littlo to the n. of Loudon. The boulders, which are the most striking feature of this bed, differ in size from a small pebble to masses many tons in weight. They arc portions of rocks of all ages. more or less worn. The older rocks, when from. a distance, are rounded, while those that have been broken from rocks in the district are more angular. These masses are scattered without order in the clay, the heaviest blocks occurring frequently in the upper portion of the bed. Nor is there any indication of their having sunk in the clay from gravity—the clay seems to have been so viscid when the materials assumed their present position, as to have successfully resisted the immense pressure of these enormous blocks. The boulders have not that rounded appearance produced by the action of water in a river-course or on the shore between high and low water marks. They have a greater or less number of rubbed faces, produced evidently by being forced. while held in one position, over the solid rocks beneath. The rubbed and scratched
surfaces exhibited on these rocks, when the superincumbent clay is removed, plainly testify to their origin. Several interesting examples of such rubbed surfaces exist in the neighborhood of Edinburgh. They have been carefully examined and described by Fleming. Chambers, 31ilne-Ilome, and other local geologists. A careful observer can determine from the scratebings the direction of the current which bore with it the rub bing boulders. In the district to which we have alluded. these indicate a (weet from the west. The general direction, however, in America, in Britain, and in Scandinavia, seems to have been from the poles towards the warmer regions of the earth.
The 13. contains no fossils strictly its own. Organisms exist in the boulders obtained from the older fossiliferous rocks; hut, no indications have hitherto been observed of a fauna or flora belonging to the period of the deposition of this bed. In the brick clays and gravels overlying it in Scotland, there are shells of arctic character.
The origin and structure of this remarkable bed have been a puzzle to geologists. That it was produced by the Noachian deluge, as was universally believed not many rears ago, finds now no supporters. The present approved explanation assigns it as the product of a glacial ocean, in which the materials were borne violently along, pressing hard upon the sea-bottom, so as to wear and scratch it. But, while there is little room to doubt that such was the general fact, it remains to be shown how a merely ice charged ocean could carry along such vast masses of clay and blocks allowing them all the time to press so hard upon the sea-bottom as to mold its whole figure—for such appears to have been its work.