_Younger beds._--That formations younger than the St. Peters sandstone once overlaid this part of Wisconsin is almost certain, though no remnants of them now exist. Evidence which cannot be here detailed[4] indicates that sedimentation about the quartzite ridges went on not only until the irregularities of surface were evened up, but until even the highest peaks of the quartzite were buried, and that formations as high in the series as the Niagara limestone once overlay their crests. Before this condition was reached, the quartzite ridges had of course ceased to be islands, and at the same time had ceased to be a source of supply of sediments. The aggregate thickness of the Paleozoic beds in the region, as first deposited, was probably not less than 1,500 feet, and it may have been much more. This thickness would have buried the crests of the quartzite ridges under several hundred feet of sediment (see Fig. 11).
[4] Jour. of Geol., Vol. III (pp. 655-67).
[Illustration: Fig. 11.--The geological formations of southern Wisconsin in the order of their occurrence. Not all of these are found about Devil's lake.]
It is by no means certain that south central Wisconsin was continuously submerged while this thick series of beds was being deposited. Indeed, there is good reason to believe that there was at least one period of emergence, followed, after a considerable lapse of time, by re-submergence and renewed deposition, before the Paleozoic series of the region was complete. These movements, however, had little effect on the geography of the region.
Finally the long period of submergence, during which several changes in sedimentation had taken place, came to an end, and the area under discussion was again converted into land.
_Time involved._--Though it cannot be reduced to numerical terms, the time involved in the deposition of these several formations of the Paleozoic must have been very long. It is probably to be reckoned in millions of years, rather than in denominations of a lower order.
_Climatic conditions._--Little is known concerning the climate of this long period of sedimentation. Theoretical considerations have usually been thought to lead to the conclusion that the climate during this part of the earth's history was uniform, moist, and warm; but the conclusion seems not to be so well founded as to command great confidence.