CONCLUSIONS AS TO SOUTH MOUNTAIN GLACIATION. 15 



2. A tongue of the glacier went over the Triassic hill south of Leithsville (how 

 far over is not known) at an elevation of 980 feet above tide. This tongue came 

 through the gap made by the opening of the Saucon valley into the Lehigh valley 

 and crossed transversely the Saucon-Durham divide. 



3. There are at least 15 feet of glacial gravel on the Lehigh-Schuylkill divide at 

 Topton, Pennsylvania, at 490 feet above tide. This gives the origin of all glaciated 

 matter in the Schuylkill valley in the Lehigh region. 



4. A section at West Bethlehem gives at least 12 feet of red clay with bowlders 

 (called provisionally "bowlder-clay " ). This exists universally to the north of the 

 South Mountain range, near Bethlehem, below a level of 440 feet above tide. It 

 may have been deposited higher, but the red clay from decomposed gneiss is so 

 similar in color that the two can be distinguished only by the foreign stones, and 

 these are not always present. Under this clay come at least 22 feet of sandy 

 gravel (''plunge and flow" structure), which rest on the scraped surface of the 

 limestone. The sequence is: (a) glacial erosion; (6j deposit of gravel; (c) deposit 

 of bowlder-clay. South of the mountain in the Saucon valley we do not find the 

 bowlder-clay. This shows no flow up and through that valley when the clay was 

 forming to the north. 



5. The moraine-stuff" consists of local angular fragments, much decomposed and 

 oxidized, mixed with water-worn and sometimes striated foreign rock (trap, 

 Oneida, Medina, Clinton, Oriskany, etc.), with polished surface ; oxidation extend- 

 ing from a few lines to one-half inch from the surface, while the interior is gen- 

 erally fresh, except where the specimen is highly porous. This represents old 

 local surface material that has been scraped and pushed ahead, mixed with recent 

 foreign material taken from a river bed. As the glacier crossed the Lehigh at 

 nearly a right angle at Bethlehem, and as the head- waters of that stream were 

 generally free from ice, we can see that the portion of the river that escaped sub- 

 glacially left its rocks, gravel and sands to be caught by the glacier and carried 

 upon and over the mountains, Sandy and gravelly till is found at all elevations up 

 to 980 feet above tide, and of all dei)ths up to 30 feet. 



G. From Allentown to Easton the glacier retreated down the Lehigh valle^^ The 

 plunge and flow gravels were probably deposited ^s the river made its subglacial 

 plunge, and the bowlder-clays when the river was dammed by the choking of its 

 subglacial channel. A lake would be formed of the Lehigh valley, with slight 

 drainage subglacially, but with its main outlet over the Lehigh-Schuylkill divide, 

 which is now covered with glaciated material to a considerable depth. This 

 divide is about four miles wide at the 500-foot contour line. 



The conclusions from the above are : 



1. That the general parallelism between the extreme line of glaciation and that 

 of the great moraine indicates an impetus from a common center. 



2. That the exposed gneiss on the mounhiin-top and fresh nature of the foreign 

 material shows a recent date to that impetus. 



3. That coming from a common origin and by a recent impetus, we are justified 

 in calling the recently found evidence a "fringe" of the great moraine which was 

 formed at the retreat of the same glacier that caused the " fringe." 



4. That until the ice had retreated north of the mouth of the Lehigh there was 

 an extensive discharge into the Schuylkill valley.* 



*See maps in Report of Progress, D3, Atlas to vols. 1 and 2, Second Geological Survey of Penn- 

 sylvania. 



