608 w. a. nelson volcanic ash bed in the ordovician 



Conditions existing at the Time of Ash Deposition 



This ash-fall thus occurred just before the close of Lowville time, which 

 was a period of much oscillation of the earth's crust and intensive warp- 

 ing. The ash does not mark an unconformity, but does slightly precede 

 the unconformity which exists in the Southern States between the Black 

 River and Trenton groups. It is interesting to note that after the ash 

 deposition the Le Ray limestone deposited to a greater thickness as one 

 goes north, there being eight feet of Lowville belonging to the Le Ray 

 time at Singleton, Tennessee, while 190 miles to the north, at High 

 Bridge, Kentucky, there is 18 feet of this same limestone. At High 

 Bridge, Kentucky, the Le Ray formation of the Lowville time is overlain 

 by about two feet of reworked ash, showing that the earth to the east or 

 southeast was probably elevated so that the ash was either carried away 

 by shore currents or washed off of the landmass to the southeast or east 

 and redeposited at the base of the shore phase of the Curdsville Sea, an 

 arm of which covered all of middle Kentucky and a narrow strip along 

 the northern part of Middle Tennessee. The conditions existing in the 

 Lowville Sea at the time of deposition of the ash are partly inferred from 

 things seen in the different sections examined. At Pikeville, Tennessee, 

 the bentonite rests on a two-inch chert layer, on the surface of which are 

 well preserved fossil ripple-marks. The parallel lines of these marks ex- 

 tend south 48 degrees east, which would show that if these marks were 

 made by a coastwise current that its direction was north 42 degrees east, 

 or south 42 degrees west, which would be roughly parallel with the shore 

 of the Lowville Sea. This ripple-marked chert has a thin surface coating 

 of brown iron oxide, and also this surface contains large bryozoan frag- 

 ments, as Avell as a scattering of Hat brachiopods. This ripple-marked 

 chert occurs at High Bridge, Kentucky, and at Birmingham, Alabama, 

 and at both places the bentonite rests on it directly. The presence of 

 such easily recognized ripple-marks at three points many miles apart 

 shows that, in this area at least, a uniform depth of water existed, and 

 the other facts noted would indicate that the sea was comparatively 

 shallow — say, not over 25 feet deep — at the time the volcanic ash fall 

 occurred. It is, of course, possible that the ripple-marks noted were 

 caused by tidal action or by currents which were not coastwise, but were 

 only local shore currents which abutted against the ancient shore 

 obliquely. The fact that the ash was deposited in water is indicated by 

 the bedding it exhibited in the road-cut at Singleton, Tennessee. At this 

 locality the bed is divided into three distinct layers. At the top of the 

 bentonite is a half -inch streak of white calcium carbonate, then 10 inches 



