Post Tertiary Deposits are scattered profusely over the district of which Birmingham is the centre, and present many problems of too complicated a character to be discussed in the pages of this guide. It must suffice to indicate a few of their chief exposures and characteristics. The term “Boulder Clay” is used in this note to denote a clay shown to be connected with the Glacial epoch, by containing a greater or less number of erratic blocks; and in the employment of the term, no theory regarding the method of formation of the deposit will be implied.
The Post Tertiary deposits of the district may be arranged in the following general order:—
I.—Lower boulder clays.
II.—Middle glacial clays, sands, and gravels.
III.—Upper boulder clays.
IV.—Post glacial clay, sands, and gravels.
The most complete section that has been found is at “California,” near Harborne.
Resting upon the Bunter Sandstone, about 480 feet above the sea level, is a Lower boulder clay, containing erratic boulders of slate, felsite, quartzite, intermixed with blocks and stones of local origin. Many of the erratics are angular, and some (especially the slates) are finely striated. The whole deposit is unstratified and compact, and the boulders are roughly pressed together, in every variety of position, without any orderly arrangement. This boulder clay is succeeded by the Middle Sands and Gravels which are irregularly stratified and show false bedding. Fragments of coal occur among the pebbles. The sands and gravels dip rapidly to the S.W., and pass under an Upper boulder clay. The Upper Boulder Clay consists of a compact mass of clay with erratics scattered through it; but the erratics are neither so abundant nor so confusedly pressed together as in the lower bed. Granite has been found, although rarely, associated with the travelled felsites and quartzites, together with a few flints; and local stones and blocks are also mixed up with the clay—the clay itself however largely preponderating and being available for brick making.
The series is capped by a mixture of clay, sand, and gravel in varying proportions, which fills many hollows that have been washed out of the upper clay; and must be regarded as Post Glacial. Taking the general divisions indicated by the California section, attention may be directed to the following illustrative facts and sections. Glacial striæ upon the surface of the rock have been noticed at Weoley Hill Quarry close to California. The removal of a mass of clay, sand, and gravel exposed a distinctly striated surface of hardened Bunter Sandstone. The polished surface dips towards the south west, in which direction the principal striæ run, although there are several cross striæ. The complete section shews (a) striated and polished Bunter (altitude 520 feet above sea level); (b) thin bed of marl; (c) sands and gravels (Middle Glacial); (d) clay (Upper Boulder Clay).
A very large number of well-marked and finely smoothed and polished grooves occur upon the blocks of native rock which are strewn over the irregularly shaped mass of basalt constituting Rowley Hill, Worcestershire. Isolated grooved blocks rest upon the surface of the hill, having been carried by external force into their present position; but there is also, at Rowley Hall Quarry, a kind of platform, capping the solid mass of basalt, which is almost entirely composed of blocks with smoothed and grooved surfaces, stiffly imbedded in clay.