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Full Text: GEOLOGIC HISTORY 471 past events in this region, wily the barest outline can be presented at this time. Gaps will be filled by future workers. The events of each era and period have been described somewhat in the descriptions of the different formations. It remains therefore to piece together the records. Due to the fact that the pre-Cambrian events are so different from those of later times it is fitting that Dr. Fraser tell the geologic story of that distant time. Pre-Cambrian History By DONALD M. FRASER With the assumption that the Franklin limestone is the oldest formation in the district, the earliest recorded event in Lehigh County would be the. deposition of a series of carbonate rocks. Little else is known regarding this earliest time. Following the accumulation of the limestone, or possibly more or less contemporaneously with it, elastic sediments containing carbonaceous material were deposited. The present quartz-graphite member of the Franklin formation is the remnant of this depositional period which surely resulted in a fairly extensive sedimentary formation at the time of its accumulation. With the changing conditions of deposition the sediments from which the present Moravian Heights formation has been derived were laid down. These were quartzose sediments but were more argillaceous than the elastic material associated with the Franklin formation. Again, on the basis of the present distribution of small areas of the Moravian Heights throughout tens of square miles, it is assumed that the original sediments were of considerable extent. They may well have covered several hundred square miles. The next formation is the Pochuck gneiss. This basic rock, in the past, has been considered as of possible partial sedimentary origin and in part to have been derived from the metamorphism of a gabbro or diorite-gabbro type of igneous material. The present writer, regards the Pochuck to be entirely of igneous origin. It may have been formed by the metamorphism of a gabbroic type of rock but it could also have been derived from the metamorphism of a series of andesite and basalt lava flows. Whichever may prove to he the correct interpretation, the next event in the district following the accumulation of the Franklin and Moravian Heights sedimentary formations was igneous activity. Either the present Pochuck gneiss material was introduced by intrusions of a basic magma or by the extravasation of lavas. The earliest of these three formations has been tentatively assigned to Archean time and the later two to the Huronian. They are all of patchy occurrence and with the exception of the .Pochuck have very small areal extent. Later Proterozoic time probably saw extensive erosion and igneous invasion of an acid type of magma. The group of rocks known as the Byram gneiss ranges from what is considered to be typical Byram (a quartz-microcline microperthite rock) to quartz monzonite types. Associated with these are numerous pegmatites. The entire series has been mapped as Byram because more detailed study and mapping than that possible on the scale of t.he map used here, will be necessary
Collection: Pennsylvania Geological Survey
Series: Fourth Series, Bulletin C 39
Book: Lehigh County Pennsylvania Geology & Geography
Imprint: Harrisburg, Pa.: Dept. of Internal Affairs, Topographic and Geologic Survey, 1941.
Section: Geologic history, Pre-Cambrian history
Author: Benjamin L. Miller
Co-Author: Donald M. Fraser;
Project Source: Lehigh University Digital Library Projects

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