Page:The Journal of geology (IA journalofgeology21894univers).pdf/122

tion than with reports of state or national surveys. At present, however, the more active state surveys offer the best approach to work of this class, and with the recently increasing interest in physiographical investigation, I hope to see chapters of their annual reports devoted to well illustrated essays of this character.^[1]

Rügen, a composite island on the Baltic coast of Prussia, has long attracted attention of geologists from the peculiar dislocations of its Cretaceous strata, in which glacial till was peculiarly involved; so that some observers concluded the dislocations were the product of pressure from the advancing ice sheet. Credner now concludes in effect as follows: The Cretaceous strata, while still horizontal, were overspread by a sheet of till. The compound mass was then, after the disappearance of the ice sheet, fractured and dislocated in a rather irregular manner, although the lines of movement in a new measure follow systematic courses. A new constructional topography was thus produced; and a significant advance was made in its subaërial denudation. Then a second ice sheet cross the Baltic, rubbed over the uneven surface of Rügen, softened its surface expression, and distributed an irregular deposit of drift over the Cretaceous beds and the older drift. Since then, a moderate depression has submerged part of the region, converting Rügen into a group of islands; and these have been in still later times soldered together by sand bars.

Concerning the evidence thus gained of a complex glacial period, Credner remarks: A convincing proof of the long interval between the two glaciations is found both in the heavy fracturing and faulting that took place between the deposition of the two northern drift formations; and in the distinct denudation that was suffered by the constructional forms produced by the dislocations, before the deposition of the second drift (p. 416, 417).