Page:History of Goodhue County, Minnesota.djvu/40

 14 HISTORY OF GOODHUE COUNTY position and also the oblique ebb and flow structure. 'S — The presence of only marine fossils, of shales, mud cracks and ripple marks. The absence of tilting and of the influence of heat shows that ttiey were never disturbed by volcanic eruptions in this quarter. The enormous amount of material deposited shows that long periods of ceaseless activity were consumed to perform such a tremendous amount of erosion. Our limestone formation, for instance, extends from New York to the Rockies. The large quantities of carbon dioxide in the Limestones is thought to have been derived from the cooling atmosphere. The sediments were hardened by their own cohesion and the infiltration of cements. The wide extent of country over which some of the formations spread indicates that when submerged it formed a wide, shallow sea bottom. The mud cracks and ripple marks speak of the beach. The compacted layers of sediments have received dif- ferent names. The lowest is the Potsdam sandstone, also called St. Croix, from the St. Croix valley, where it outcrops. Over this are spread in succession : J. 2. the Si. Lawrence and Jordan sandstones, by some included in the St. Croix, which can be seen in outcrop in Barn bluff, College hill and Twin bluff; 3, lower magnesian limestone, capping our bluffs; 4. New Richmond sandstone at Cannon falls: 5, Shakope limestone, seen at Cannon falls and Northfield; '». St. Peter sandstone^ seen at White rock, Castle rock and the falls of the Cannon; T. Trenton limestone, seen at Wanamingo and Cannon falls. "Ai some period life appeared. Some geologists place this evenl hack as far as 2< )..0 years, so we have a wide range of lime to choose from. The archaean rocks contain no clear traces of life, so thai the rocks in our immediate neighborhood are of special interest, as they introduce us to some of the earliest known animals. Other living things, such as lichens and mosses, may have preceded them, hut owing to their delicate structure, the physical conditions under which they lived and the lone- period of time that has elapsed, no definite traces have been left. The St. Croix formation contains traces of plants regarded as sea weeds. There is no evidence of land plants. In the geologic foundations of the county are evidences of all the types known to the animal kingdom except the vertebrates. The combined contribution of animal life to the rock structure of our present home was great. The successive generations were piled up on the tombs of their ancestors, while those whose shells were reduced to powder — and their number was undoubt- edly far greater than those whose remains are unearthed today — furnished the material for the encasing rock. In general it is an accepted proposition that limestones deposited by marine agencies were produced by the calcareous remains of animals