256 Field Columbian Museum — Geology, Vol. 1. 



Data are meagre for estimating the length of this period. Prestwich* 

 has estimated the rate of erosion by the Thames as one inch in 1,000 

 years. The chalky Cretaceous and Oolitic strata over which the 

 Thames flows are doubtless eroded at a more rapid rate than the com- 

 pact limestone in which Wyandotte cave is situated. Taking this 

 rate, however, as a minimum, it will be found that a period of 360,000 

 years would be required to erode the "Senate Chamber" to the 

 depth of the base of the stalagmite. 



MARENGO CAVE. 



The Cave Floor Terrace. — The greater portion of the floor 

 of this main cave shows a well marked terrace recording two distinct 

 stages in the life of the stream which before its final disappearance 

 flowed through the cave. Of these two stages the stream of the 

 older stage had a width of from 15 to 20 feet and a current of 

 sufficient velocity to make large ripple marks on its bed of coarse 

 alluvium. These ripple marks are symmetrical and their long 

 slope is plainly away from the present entrance to the cave. This, 

 therefore, was the direction of flow of the stream. In its second 

 stage the stream was reduced to a width of about 10 feet and its cur- 

 rent was more sluggish. It cut a trench of the above width with 

 nearly vertical walls to a depth of about two feet in the bed of the old 

 stream, but did not have a current of sufficient velocity to produce 

 ripple marks on its bed. A further greater sluggishness as compared 

 with the first stream is indicated by the somewhat winding course 

 which it took through the bed of the latter. The disappearance of 

 this stream must have taken place somewhat suddenly, for there has 

 been no trenching of its bed nor sloping of its banks such as would 

 have occurred if the flow of water had diminished gradually. The 

 bed, now quite dry, has a slightly concave form. A draining away of 

 the stream by the opening of new conduits at a lower level seems the 

 most natural explanation of its two stages and final disappearance. 

 It is of course not impossible that these stages mark a diminution in 

 rainfall or supply of waters from above, but there is on the whole 

 little reason to suspect such abrupt changes in these conditions. It 

 is not unlikely that the large spring situated a few rods west of the 

 present entrance represents the present point of issue of the stream. 



*Geology, vol. i, p. 107. 



