The interruption of this cycle was caused by the advent of the glacial period which disturbed the normal course of erosion. This period was accompanied and followed by slight changes of level which also had their influence on the streams. The consideration of the effects of glaciation and of subsequent river erosion are postponed, but it may be stated that within the area which was glaciated the post-glacial streams have been largely occupied in removing the drift deposited by the ice from the preglacial valleys, or in cutting new valleys in the drift. The streams outside the area of glaciation were less seriously disturbed. At levels other than those indicated, partial base-levels are suggested, and although less well marked in this region, they might, in the study of a broader area, bring out a much more complicated erosion history. As already suggested, one cycle may have preceded that the remnants of which now stand 1,000-1,100 feet above sea level, and another may have intervened between this and that marked by the 800 to 900 foot level. From the foregoing it is clear that the topography of the region is, on the whole, an erosion topography, save for certain details in its eastern portion. The valleys differ in form and in size, with their age, and with the nature of the material in which they are cut; while the hills and ridges differ with varying relations to the streams, and with the nature of the material of which they are composed. _Striking Scenic Features._ In a region so devoid of striking scenery as the central portion of the Mississippi basin, topographic features which would be passed without special notice in regions of greater relief, become the objects of interest. But in south central Wisconsin there are various features which would attract attention in any region where the scenery is not mountainous. [Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. XXI. Cleopatra's Needle. West Bluff of Devil's Lake.] [Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. XXII. Turk's Head. West Bluff of Devil's Lake.] [Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. XXIII. Devil's Doorway. East of Devil's Lake.] On the bluffs at Devil's lake there are many minor features which are sure to attract the attention of visitors. Such are "Cleopatra's Needle" (Plate XXI), "Turk's Head" (Plate XXII), and the "Devil's Doorway" (Plate XXIII).