FOSSIL REEFS: SILURIC 419 



older days quarries were opened in these reefs, but, owing to the 

 absence of stratification planes or seams, the operations were more 

 difficult and costly, and the old quarries were abandoned when the 

 bedded strata were discovered. 



The exposed face of the old quarry is generally thoroughly 

 weathered, and it shows an all but structureless surface, the chief 

 characteristic of which is the total absence of stratification. The 

 reefs consist largely of stromatoporoids, among which the genus 

 Clathrodictyon seems to abound. The reefs as exposed are gen- 

 erally several hundred feet in diameter, with a height of perhaps 

 50 feet. They thus constitute a series of independent reef mounds 

 rising from a platform which is sometimes composed of organic 

 accumulations and at others of clastic material. On the borders 

 of the reef mounds the material consists of coral sand, consolidated 

 into calcarenyte, and this dips away from the reef in all directions. 

 This dip is not due to any tectonic deformations, but is the original 

 dip of the clastic material on the flanks of the reef. The dips 

 observed in the Shoonmaker quarry near Wauwatosa range as high 

 as 54 degrees close to the reef, while dips of 28 to 34 degrees 

 are more common, and others as low as 18 degrees occur. Further 

 away the strata become practically horizontal. (Grabau-40.) At 

 the Distillery quarry in Milwaukee (foot of Twenty-ninth Street) 

 dips of 40 degrees toward the east and west and of 20 degrees and 

 more to the south have been observed. The two reefs mentioned 

 and a third in the grounds of the National Military Asylum form 

 a triangle, in the center of which quarries have been opened to a 

 great depth in evenly bedded, rather fine-grained calcarenyte, this 

 being the coral and crinoid sand derived from the reef and swept 

 together in even-bedded strata in the inter-reef spaces. These 

 limestones are sparingly fossiliferous, the roving orthoceratitic 

 cephalopods alone occurring in abundance. The strongest possible 

 contrast existed between the reef mounds, on the one hand, on the 

 borders of which flourished a rich fauna of corals, bryozoans, 

 brachiopods, molluscs, trilobites, and crinoids, and the nearly bar- 

 ren sandy bottoms of the comparatively shallow sea on the other; 

 desert areas surrounding luxuriant oases of animal and plant life. 



A typical reef mound exposed by quarrying operations near 

 Cedarburg, 20 miles north of Milwaukee (Grabau-40), shows a 

 thickness of about 30 or 40 feet of coral rock, though the full 

 measure is not exposed. The diameter of the old reef mound was 

 perhaps 300 feet from north to south. Much of the reef rock has 

 become dolomitized, and in many of the bedded strata the corals 

 have been dissolved out. The central part of the reef consists of 



