350 ARTHUR C. TROWBRIDGE 



freshness and apparent youth of Devils Lake gap, and its size, 

 harmonious with the dimensions of the Upper and Lower Narrows, 

 suggest that these three gaps were cut at the same time and during 

 some post-Silurian period. And yet Devils Lake gap may have 

 carried a large river in pre-Cambrian times and the gap may have 

 been reoccupied at a later time when the two Baraboo narrows 

 were cut. 



Whether the pre-Cambrian Devils Lake gap was cut by a single 

 intrenched meandering stream, or by two streams with a col between 

 them, the depression must have been a deep one. The rim of the 

 canyon is today represented by the 1,470-foot summit plain, and the 

 pre-Cambrian rim may have been higher than this. Cambrian 

 sandstone in the gap is known as low as 900 feet A.T. The depres- 

 sion must therefore have been at least 570 feet deep in pre-Cambrian 

 times. Certainly if the gap was cut by the same stream which 

 reduced the inclosure between the ranges to 340 feet A.T., and 

 probably if it was cut by two streams tributary to main drainage, 

 the bottom of the gap at the end of pre-Cambrian times was 

 not much higher than 340 feet A.T. The pre-Cambrian gap was 

 then probably not far from 1,100 feet deep. The whole district 

 at this time is known to have had a relief of 1,200 or 1,300 feet. 



PALEOZOIC HISTORY 



The Paleozoic, marine, sedimentary rocks of the district 

 record the second important step in the known history of the 

 district. 



Upper Cambrian sandstone is found on the ranges to an altitude 

 of 1,200 feet A.T. and probably exists at slightly higher levels. 

 On Wood's Quarry Hill, 3 miles northwest of the lake, rounded 

 pebbles of quartzite are found in the uppermost beds of the Madison 

 formation where it is overlain conformably by Ordovician dolomite, 

 giving evidence that quartzite was exposed on near-by land up 

 until the very end of the Cambrian period. In the light of these 

 facts, it is believed that the South Range was an island in the 

 Cambrian sea, and that the Devils Lake gap was not filled to a 

 higher level than 1,020 feet, although the sea seems to have reached 

 at least to 1,200 feet. 



