320 
prevailing at the western bluffs with those on the eastern border. At the 
north the difference in elevation is 300 feet, in the central portion about 
the same, and in the southern portion 260 feet. 
GEOLOGY 
A study of Plate LX XI, which was traced directly from the map of 
the State Geological Survey, will give a very good idea of the geology oi 
this region from Thebes to Chester and west to the third principal 
meridian—the boundaries of the map. The oldest rocks are placed first 
in the legend. As we proceed eastward from the Mississippi we encounter 
successively younger strata, those laid down later in geological history. It 
should also be kept in mind that this whole region experienced periods of 
submergence beneath the sea, with a deposition of limestones, sandstones, 
or shales, depending upon the conditions of the inland waters, these 
periods of submergence alternating with periods of elevation and sub- 
sequent erosion down to base level so that where new layers of sediment 
were deposited upon the old eroded land forms, there are many examples 
of what the geologist calls “nonconformity.” 
The Ordovician and Silurian, which are the oldest rocks exposed by 
erosion in Union and Alexander counties, lie like a thin wedge between 
the Quaternary deposits on the west and the Devonian on the east, run- 
ning north and south from Fayville to Reynoldsville, and consist mostly 
of limestones, sandstones, and shales. Since these Pre-Devonian rocks 
have been fully described by Savage (’08, ’09, 10), any attempt at a full 
description would be out of place here. 
The Devonian may be divided into the Lower, Middle, and Upper, 
and Savage (’20) says that the total thickness of the Devonian rocks in 
southwestern Illinois is not less than 800 feet. The lowest series of the 
Devonian is the Helderbergian, represented by Bailey limestone and Back- 
bone limestone. The high bluff facing the Mississippi from Harrison 
Creek south to Thebes, which one passes in driving from Jonesboro to 
Reynoldsville along the Cape Girardeau road, may be classed as Helder- 
berg limestone. The Clear Creek chert is a characteristic layer of the 
Middle Devonian and may be found in two forms: in limestone, as 
nodules from one inch to two inches in thickness; and as a regular forma- 
tion constituting the axis of such elevations as Hamburg Hill, Atwood 
Ridge, Pine Knob, and Dago Hill, extending down through the Jonesboro 
quadrangle to Tamms. The gravel pit at Tamms is in this formation, 
and the silicon which is mined at Delta, in Alexander county, is in the 
Clear Creek chert which has disintegrated. We speak of it because it 
outcrops in the beds of so many streams, such as Clear Creek, and others, 
and is an important road-builder and soil-former. Overlying the Clear 
Creek chert is the Dutch Creek sandstone, so called because it is best 
