CENTRAL STABLE REGION OF THE UNITED STATES 



45 



H /AG ARAN 



Fig. 5.7. Hypothetical section of the Mackinac Straits region showing collapse formations above the 

 Niagara limestone, and the breccia chimneys and stacks. Reproduced from Landes, 1945. 



tOo 



Fig. 5.8. Isopach map showing aggregate thickness of Salina salt. Reproduced from Landes, 1945. 



shales is 4800 feet below sea level. As previously explained, the Eastern 

 Interior basin was part of a depression that included the Michigan in pre- 

 Devonian time, but from then on the two basins sank separately, leaving 

 the Kankakee arch between. 



The La Salle anticlinal belt (see Fig. 5.9) is a row of anticlines 

 arranged en echelon, and it extends over 200 miles from north central to 

 southeastern Illinois. The north end of the en echelon belt may be con- 

 nected with the east-west trending Savanna-Sabula anticline ( Eckblaw, 

 personal communication), which extends into eastern Iowa. The south 

 end may merge with the Wabash River anticline. The La Salle anticlinal 

 belt formed chiefly during the Pennsylvanian period and divided the pre- 

 Pennsylvanian Illinios-Indiana-Kentucky basin into two parts, the larger 

 and western of which is generally known as the Illinois basin. The Oak- 

 land anticline borders the La Salle closely on the east. 



The first deformation took place in post-Chester, pre-Pennsylvanian 

 time (Fig. 5.10). Further deformation continued during the Pennsylva- 

 nian progressively southward. In La Salle and Douglass counties at the 

 north end, the early movements were the greatest, and the crest of the 

 anticline was elevated 900 to 1400 feet above the adjacent basins. In 

 Lawrence and Wabash counties to the south, the greatest movements 

 occurred within the Pennsylvanian. Since the Pennsylvanian beds are 



