THE HISTORY OF THE WISSAHICKON

In some sense, the history of the valley of Wissahickon Creek could be said to be the early history of the city of Philadelphia. The name was taken from a Delaware Indian word for the valley, used in pre-colonial times. Settled in the late 17th century, by the early 19th century there were 54 mills erected along the stream, the source of power for the growing city. The valley is the type locality for the Wissahickon schist which underlies the city, a foliated metamorphic rock with predominant mica and quartz, which provided a building material for early inhabitants.

The Wissahickon entered the geological literature in the earliest 20th century. Since that time, virtually all of the tools of geology have been brought to bear on the questions of its origin and age. The rock itself remains enigmatic. This series of papers will explore the work done during the 20th and 21st centuries. Those first geologists benefited from several centuries of investigations of earth materials. By the 20th century, much was known. Minerals of interest for their beauty and/or usefulness had been identified since antiquity. By the last decades of the 18th century there were reliable and organized means of cataloging their physical properties. Those means became increasingly sophisticated, with numerical scales attached. Devices such as the sclerometer to test hardness were originated in the late 18th century, and improved throughout the 19th. By the end of the century it was recognized that different methods of testing hardness were not congruent because different instruments tested different atomic properties. Optical methods improved with the origination of thin sections and Nicol prisms. Fustion points were determined with greater and greater accuracy, from use of the blowpipe through Seger cones to Joly's meldometer. More and more chemical elements were discovered, and mineral compositions and their properties were clarified. Minerals were increasingly identified with particular types of field sites. As well, there was better understanding of the interlocking nature of magnetism and electricity. Wissahickon researchers took advantage of all possible tools, and their work was the impetus for further development.