freshwater low-level drifts of the Avon and Lower Severn, must have been first formed, as it is merely a continuation of the gravel-beds upon which the upper strata of Strethill repose. The following is a list of the rocks occurring in the drift between Ironbridge and Shrewsbury. I have avoided naming them before, because the list applies equally to every bed composing the drift, and from careful observation I cannot find that the constituent rocks of the water-worn gravels differ from those in the clay-bed, or the larger masses from the smaller materials. The only constant fact that impresses one is the very large proportion of the constituents that appear to be of local origin, the variations of colour and character being dependent on purely local circumstances. All who have noticed the superficial gravel-deposits distributed throughout the kingdom cannot fail to trace their variations of colour, &c., to their evident derivation from neighbouring formations. The orange-coloured gravels about London and in the south-east of England probably derive their colour from the ferruginous beds of the Greensand formation, and a similar relation of colour and material can be traced between the New Red Sandstone and the gravels that rest upon it. In the following list of the rocks of the Severn-Valley drift, I have given the nearest direct distance of the parent formation in situ. List of Rocks occurring in the Drift of the Severn Valley. Flints, fragments of hard chalk, and chalk Corals in small numbers (80 miles to the S.E., 100 miles to the N.E.). Oolitic limestone, a fragment (from 40 miles to the south). Lias fossils, including Cardinia ovalis from the Lower Lias, Gryphaa incurva, &c. (20 miles to the north, and 30 miles to the south). Bunter sandstone in fragments and as sand (4 miles to the N.W. and 4 miles to the S.E.). Permian sandstone (2 miles to the S.E.). Coal-measure rocks and fossils, drift coal, and nodules of ironstone in large quantities ( mile to the S.E.). Mountain-limestone in large masses (2 miles to the north). Wenlock shale Trilobites (Phacops longicaudatus), Corals, and Shells (immediately adjacent). Wenlock limestone and characteristic Shells and Corals (mile to the S.W.). Llandovery sandstone and Caradoc sandstone abundant (3 miles to the west). Cambrian rocks (12 miles to the west). Greenstone abundant (evidently derived from the Wrekin, 3 miles to the N.W.). Various granites from Cumberland (120 miles to the north), and perhaps from Scotland. Porphyritic syenite, probably from the Maiverns (35 miles to the south). Trap-ash. I am indebted to Dr. Duncan, Sec. G.S., and to Mr. Jenkins, Assistant-Secretary G.S., for the accompanying list of derivative fossils from the Strethill drift-beds, which they have kindly determined for me. List of Fossil Corals found in the Drift of the Severn Valley. 1. CRETACEOUS (upper). Parasmilia centralis, Edwards & Haime serpentina, Edwards & Haime 2. MOUNTAIN LIMESTONE. Zaphrentis Phillipsi, Edwards & Haime Cyathophyllum Wrightii, Edwards & Haime 3. SILURIAN. No. of Specimens. 4 93 Palæocyclus rugosus, Edwards & Haime. Dudley & Wenlock...... 4 Fletcheri, Edwards & Haime. Dudley Plasmopora petalliformis, Lonsdale, sp. Dudley Ptychophyllum patellatum, Schl. Wenlock Cyathophyllum pseudo-ceratites, M Coy, sp. Wenlock.. 1 1 1 3 3 4 3 1 1 truncatum, Edwards & Haime. Wenlock, Aymestry angustum, Lonsdale. Wenlock... Monticulipora pulchella, Edwards & Haime. Wenlock. lens, M Coy. Lower Silurian Halysites catenularia, Edwards & Haime. The indeterminable specimens are mostly Silurian, but there are both Carboniferous and Cretaceous forms amongst them. List of Fossil Shells, &c., found in the Drift of the Severn Valley. Productus scabriculus, Martin. Carboniferous Limestone 1 1 Sigillaria. Coal-measures Stems of Encrinites. Carboniferous Limestone. Several specimens. 3. SILURIAN. Pentamerus linguifer, Sow. Wenlock.. 3 Atrypa reticularis, Linn. Wenlock. Many specimens of all sizes. On the west side of the Severn Valley, above Buildwas, the drift rests on the flanks of the Wenlock-limestone hills to an altitude of 200 feet above the river, and upwards of 300 feet above the present sea-level, but in lesser masses than at Strethill; for the most part it consists of well-washed gravels and sands, varying much in size and character, and between which and the tough clay-bed of Strethili I have been unable to trace any close stratigraphical correspondence; N.E. Erratic Boulders of grey granite. indeed, as far as I have been able to observe, the mounds of drift which occupy so much of the valley up to Shrewsbury consist mostly of such beds of sand and gravel as occur at the base and the summit of Strethill, the middle tough claydeposit being but slightly represented. In the cutting opposite Buildwas Abbey and in the gravel-pits near the station, the clean sea-sand, similar to that at the base of Strethill, is, however, readily distinguishable from the upper beds, which are greyer in colour and rather more clayey in composition, and may possibly correspond with the middle beds of Strethill, though much less tenacious. Descending the river to Bridgnorth, and, for the present, confining our observations to the valley, the drift ascends to an altitude of 170 feet above the river (which is here 30 feet below the river at Buildwas) or about 240 feet above the sea. On the east side of the valley the drift is disposed in two wellmarked terraces, their sides sloping at an angle of 30°, the lower at an altitude of 43 feet above the valley-alluvium, and the higher 43 feet above the lower terrace, and at altitudes of 113 and 156 feet above the sea. The exact correspondence in height of these two successive terraces of erosion is rather remarkable; and as they have all the appearance of old river-banks, it has been suggested by my friend the Rev. Mr. Wayne, of Wenlock, that they may mark two distinct periods of interruption in the elevation of the land-a supposition which is well sustained by the structure of the crest of the upper terrace, a section of which, in the St. James's gravel-pits, exposes at one point a regular pebble-ridge, or estuary coastbeach, composed of rounded boulders with but a slight intermixture of smaller matter. The bulk of these terraces consists of an even and stratified intermixture of gravel and small rounded boulders derived from a great variety of formations, including the following: S.W. Fig. 2.—Section across the Valley of the Severn one mile below Bridgnorth. Sea-level a. Drift Gravel. b d. Bunter Sandstone. e. Permian beds. p List of Rocks, &c., from the Drift in the Valley of the Severn at Bridgnorth. Llandovery sandstone. Wenlock limestone. Drift coal. Coal-measure rocks and Penneystone ironstone. Red and grey Permian sandstone. Bunter sandstone. Basalts and greenstones. Red, grey, and white granites. Quartz-rock. Elvan or compact granite-rock. A Lias fossil (Gryphœa incurva). The composition of the whole of these Severn-Valley gravels is very similar, and only differs in respect of the formation upon which they immediately rest, which always preponderates; thus, the Bridgnorth drift partakes largely of the components of the New Red Sandstone, especially the beds at Quatford, about two miles below the town, which are almost entirely composed of the detritus of the Bunter sandstone, and contrast strikingly with the tough clays of the Strethill deposit, which I cannot help thinking have been largely derived from the waste of the Coal-measure clays and Wenlock shale in its immediate neighbourhood. The cuttings of the Severn-Valley Railway below Bridgnorth, on the west side of the valley, expose several sections of drift; gravels and sands, containing a few broken fragments of Shells, generally rest on the older formation, and in some places these are again overlain by a more clayey deposit, which may possibly be related to the clay-beds of the Strethill drift; but as the whole of the drift-beds exhibit so much variety of character, the correlation of their constituent strata in different localities is most difficult and uncertain. In describing the strata of Strethill, I mentioned that the larger constituent materials were the same throughout, but that the character and aspect of the strata differ considerably, the middle beds being composed of unstratified clay, silt, and muddy gravel, whilst the upper and lower strata consist of clean sand and water-worn shingle, evenly stratified, and without the slightest trace of mud. That the whole mass was derived from similar sources appears probable from the identity of the constituent rocks and boulders, and its variety of character and aspect must be attributable to the different circumstances under which its component strata were deposited. The long-continued action of the tide upon a particular level will tend to the removal of all fine matter capable of being held in watery suspension, and leave the coarser particles as clean sand and shingle similar to what we find on our more exposed coasts; and I would suggest that such beds, forming the base and summit of the Strethill drift, may represent periods of comparative cessation in the submergence of the land, during which fresh erosion and accession of new materials would be checked, and the débris already accumulated on the coast subjected to the long-continued cleansing action of the waves above described, thus converting it into clean sand and rounded shingle. With regard to drift clay-beds, rapid submergence would continually present fresh points of approach for the eroding waters, and might tend to the accumulation of débris at a greater rate than the finer matters could be cleansed away by repeated tidal action. The Strethill clay-bed, which was probably deposited in deeper water, contains a much larger proportion of angular and unworn fragments than the shingle accompanying the clean sand, and also lumps of soft Wenlock shale, which, had they been subjected to long coast-action, must soon have been broken up. I would submit, therefore, that the alternating strata of Strethill present us with evidence of unequal or interrupted action in the submergence of the land, in the same way that the Bridgnorth terraces prove an irregularity in the rate of upheaval. I believe that the whole of the drift-strata I have described, excepting, perhaps, the flat valley-alluvium, were deposited in the interval between a single period of depression and re-elevation, but that subsequent oscillations of level, represented by a separate series of drifts of a different character at a lower level, are also evident. The lowest beds of the Shropshire drift are nearly 100 feet above the sea-level; they are, excepting the flat alluvium bordering the river, exclusively of marine origin, entirely devoid of Mammalian remains, and, considering their immense thickness and range of altitude, their monotonous character and paucity of organic remains contrast strikingly with the drifts of lower levels, such as those of the valleys of the Somme, the Avon, and the lower part of the Severn Valley. All of these, from the smaller scale of their constituent strata and close alternation of marine and freshwater beds, appear to have been the result of less extensive variations of level than that involved in the formation of the Strethill drift, and may have gone on during comparatively short periods within the 100 feet separating the present sea-level from our lowest Shropshire drift, and subsequently to its last denudation. They would be more essentially valley-drifts than those now described, which belong to an epoch in which nearly the whole land-surface of Great Britain was probably submerged. In the valley about Bridgnorth, and on the terraces of drift before described, there here and there occur large boulders of grey granite, Cambrian slate, Llandovery sandstone, Coal-measure rocks, Carboniferous limestone, New Red sandstone, New Red conglomerate, Devonian sandstone, and a volcanic rock, probably from N. Wales. They are for the most part subangular, and, although they do not exhibit any well-marked glacial striæ, the large weight of some of them (from one to two tons) clearly implies some form of iceaction as a means of transport. These erratic blocks occur sparingly amongst the drift all up the valley to Shrewsbury, but much more abundantly on the higher ground at altitudes of from 400 to 800 feet above the sea, where they are not always accompanied by the ordinary drift. |