Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/120

82 &2 8. V. WOOD, JUN., AND F. W. TIARMER ON THE [n the Quarterly Journal of the Society for 1800 (vol. xxv. p. 446), we gave a section of the valley of the Yare at Norwich, disclosed by the sewer-works of that city then in progress. This section, which crosses the Yare valley about 4 miles west of the point where the chalk floor rises into viow above the water-line of the rivers, was very perplexing to us at the time ; and we were at first drivon to suppose that the deep hole in the Chalk in which the Middle Glacial sand was found in association with the chalky clay beneath the river-level (so out of their usual place) was due to some exceptional erosion by glacier or iceberg agency. A study of the way in which the East- Anglian valley-system has arisen, how- ever, has satisfied us that, whatever may have been the eroding agency, there is nothing exceptional in this occurrence, and that it harmonizes with the general structure indicated by the uncon- formable relation of the Lower to the Middle and Upper Glacial deposits. Indeed, in a foot-note to the paper just quoted, we pointed out that the erosion under consideration was connected with the con- version of the Lower Glacial formation into land, and the occupation of the valleys in it by ice prior to its depression beneath the sea which deposited the Middle Glacial. We have here reproduced that section (fig. 4), because it is essen- tial to the proper description of the subject-matter of this paper. The dotted lines indicate what we considered at the time, and still consider, 10 have been the excavation of the Yare Valley subse- quently to the deposition of the Contorted Drift, and prior to that of the Middle Glacial sand. Whether the chalky clay (a) associated with the Middle Glacial sand in the hole beneath the level of the river be the Upper Glacial clay, or a bed of clay precisely resembling it, which occasionally occurs beneath the Middle Glacial sand in some valleys (and amongst them the upper valley of the Yare, as shown in section V., p. 85) is immaterial to the subject, because it leaves the excavation of the val- ley subsequently to the deposition of the Contorted Drift unaffected, the clay in question, whatever it be, being an entirely different material from this drift, which everywhere in the neighbourhood around is a red semistratified brick-earth. The sand and clay (a and 6) were so bedded against each other in the sewer-excavation, and so flooded with water, that we could not satisfactorily learn which overlay the other ; but, from such information as we could obtain, we understood that their relative position was as represented in the above section : we have, however, since been told by one of the work- men that the clay was over the sand, in which case it would be merely the ordinary Upper Glacial clay (6) of the high ground. No doubt, what thus appears to be a hole below the river-level * close to the sewer-shafts. The excess of the vertical over the horizontal scale of the section greatly exaggerates the slopes ; but in the case of the sewer-exca- vation the perpendicularity may be owing to the Chalk, where the line of section traverses the interglacial valley, having formed on one side of it a cliff.
 * The left side of the hole is drawn perpendicular, as the Chalk was very