The question has been raised whether, since the basalt readily develops joints, these grooves may not be rudimentary joints, or whether disintegration may not have taken place along certain lines which have gradually become grooves. I entertain no doubt, however, of their glacial origin. No other explanation than that the grooves were the work of moving ice can account, I think, for their excellent polish, their frequent parallelism, their adaptation to the hollows and protuberances of the blocks they cross, and their predominant trend from north-east to south-west. The absence of erratics from the boulder clay in which the grooved basaltic blocks are embedded is evidence of local ice action at Rowley Hill. It is notable also that angular blocks of basalt from Rowley Hill have been found in Birmingham, blocks which must have travelled at least six miles.
It is often difficult to decide the precise age of the boulder clays of the district; and whether any individual bed is referred to the upper or the lower series is more dependent upon the glacial theories that may be adopted than upon any observations that can be made in the field. A Boulder Clay, of a typical kind, has been exposed at a brickyard, at the bottom of Oak Street, Wolverhampton. This clay contains an extraordinary number and variety of erratic blocks, without question from the Lake District and south of Scotland, a few flints, together with pebbles from the Bunter beds. One of the sides of a boulder of felsite, measuring 11 × 3 × 3ft., is flat and smooth, and covered with parallel striæ. The sands and gravels rising in small hillocks near the Cemetery, and slightly covering the clay of the pit, are probably Middle glacial. A boulder clay, formerly exposed at Icknield Street, Birmingham, while presenting the same physical characteristics as the Wolverhampton clay, differed from it widely in the nature of the embedded erratic blocks. Instead of having travelled from the Lake District or Scotland, a large proportion were derived from rocks that occur in situ at the Berwyn and Arenig Hills. The condition of the New Red Sandstone ridge, against which this boulder clay rested, was remarkable. The sandstone rock was broken up, and large fragments of it were lifted out of their position and thrust into the middle of the drift.
The changes of level which occurred during the glacial epoch are shown by the deposits at Frankley Hill. In the clays and sands cut through by the Halesowen Railway only a few erratics (felsites) were found; but on the summit of the section they are abundant and of large size (e.g., 4 × 4 × 2ft.) Professor Bonney, who has examined them, feels certain they must have come from Wales, having seen nothing like them in the Lake District. Their height is nearly 800ft. above the sea level. Were these erratics brought by land ice, the alteration in the physical geography of the country must have been enormous to have enabled a glacier to have moved downwards over this point; were they dropped from icebergs, the land must have been depressed to the extent of at least 900ft., to form a sea in which the bergs could have floated.
Turning to the Middle Glacial Clays, Sands, and Gravels, these may be seen more or less developed in almost every section cut through undulating ground; and they are occasionally twisted and contorted. In the immediate neighbourhood of Birmingham they are not fossiliferous.
At Ketley, near Wellington (Shropshire) however, fossiliferous sands and gravels occur, which I am inclined (provisionally) to assign to this period. They rest upon a bed containing erratic blocks of granite, and other rocks of northern origin; and I collected from them 13 species of mollusca. Only one species was peculiarly northern (astarte borealis) but all in the group have an arctic range of habitat. The elevation of these beds is about 357 feet above the sea.
At Fox Hall Field, New Lodge, Lilleshall (Shropshire) in a pit worked for sand, 463 feet above the sea, Mr. Woodward discovered 21 species of mollusca, three—viz., Dentalium abyssorum, Natica affinis, and Astarte borealis—being characteristically arctic and extinct in British waters.
The Upper Boulder Clay is worked for bricks in many localities. It is distinguished from the Lower Boulder Clay, by having erractic blocks sparsely scattered through it. It is often very compact and tenacious. No fossils have as yet been found in it; unless indeed a clay derived from a drain in a street at Wolverhampton, in which I detected fragments of Tellina balthica, the spine of an Echinus, Polymorphina lactea, and Polystomella crispa, may be assigned to this division of the epoch.
The extraordinary dispersion of erratic blocks over the surface of the ground remains to be noticed, and constitutes one of the most remarkable phenomena in local glacial geology. I distinguish the boulders resting on the surface of the ground from those embedded in the clay beds, although it is of course possible and probable that the clay has been largely denuded, and the boulders have thus been left exposed. Many of these erratic blocks may therefore belong to the Lower Boulder Clay; while others may have fallen from the icebergs which during the proved subsidence of the land must have floated over the “Midland” sea, and have been deposited in the Upper boulder clay, while it was in process of accumulation.
How far the dispersion of erratics over the Midlands may be referred to the ice sheet of some geologists, or to the icebergs of the Archipelago period in the history of Great Britain, must, at present, be regarded as an open question.