28 PROCEEDINGS OF THE INDIANAPOLIS MEETING. 



Custer county, Nebraska, than in any other locality which has come under mj'- ob- 

 servation. Here there may be a score of them to the square mile. In other parts of 

 the Great Plains they are few and widely separated. They vary from one acre to fifty 

 acres in area. 



Are there no outlets whatever for the surface flow of water from these depressions ? 

 There may be, but the moment that occurs the type is destroyed. The outlet deepens 

 to a ravine, the ravine to a canon, the canon opens into a valley, and so on to the sea ; 

 the primitive surface of construction has been captured and converted into a surface 

 of erosion. This process is constantly active. The chisel of water-sculpture is for- 

 ever hacking away at the remnants of the table-lands. Their edges are gashed with 

 fresh ravines, and here and there a canon pierces the very heart of the plateau. 



But the resistance to the encroachments of water-sculpture is considerable, and the 

 manner of resistance is obvious. So long as the lagoons are not filled to the brim 

 there is no chance for any " wash " to get a start. Should there be a great increase 

 of the rainfall, so that precipitation should exceed evaporation, the lagoons would fill 

 up and overflow, and the table-lands would rapidly melt away. Their preservation 

 is therefore good evidence of constancy in climatic conditions during the whole period 

 since this lake bottom became dry. At least it is conclusive evidence that there has 

 been no great variation in the direction of increased rainfall, though there may have 

 been greater aridity. These curious structural forms constitute a sort of weather 

 record which runs far back into the past. It was dry enough when Lake Cheyenne 

 was spilled out of its bed by upheaval to evaporate the remnants of that lake in the 

 lagoons, and it has since been dry enough to keep them from filling and overflowing. 

 They even give us a glimpse of the climate which prevailed in a period far more re- 

 mote, as we shall see when we inquire into their origin. 



To this question of the origin of the lagoons the most queer and contradictory an- 

 swers, ranging all the way from wallowing buffaloes to spouting volcanoes, may be 

 elicited from the old settlers. The generic relations of the lagoon type are clear 

 enough : it is a structural form unmodified by erosion. But among structural forms 

 is this an example of the sedimentary, the igneous, the coralline, the glacial, or the 

 eolian type, or is it a combination of some of these ? The title of this paper implies 

 that it is sedimentary. But sedimentation tends to produce horizontal planes. If 

 there are exceptions, such as torrential cones and sloping beaches, they have obvi- 

 ously no application to the case in hand. Yet the materials displaying this structural 

 form are indubitably lake sediments — Tertiary marls. Their unique form must, there- 

 fore, have been influenced by forms of surface already in existence when this region 

 became a lake. None of the familiar accidents of upheaval, tilting, folding or fault- 

 ing to which horizontal sediments are subject will account for such forms as these. 

 Igneous action produces lofty cones, craters, geyser basins, dikes, bosses, laccolites, 

 and sheets of extruded lava which may present considerable irregularities of surface. 

 Some of these igneous forms of construction, if they were mantled over with a sheet 

 of lake sediment, might give a result something like the lagoons and rounded hills of 

 the table-lands of Custer county, but there is no reason to suspect that any sort of 

 igneous agency has been concerned in the matter. The hint contained in the identity 

 of the popular name, lagoon, with that which designates a prominent type of coralline 

 structure is only misleading. The promiscuously irregular forms of the glacial drift 

 are more promising. Hillocks, kettle holes and morainal lakes might possibly assume 

 a facies not unlike the forms in question, at least with the help of a thin cover of fine 

 sediment; but the region is clearly beyond the recognized limits of glaciation, and 

 no drift is found either on the surface or beneath it. 



