Map Lovers’ Monday: Philadelphia Geology, 1881

FULL TITLE  Second Geological Survey of Pennsylvania, J. P, Lesley, State Geologist. Geological map of the Philadelphia Belt of Azoic Rocks, the eastern end of the Limestone Valley of Chester County, the southern edge of the New Red Sandstone of Bucks and Montgomery, and the Delaware Valley Gravels. By Charles E. Hall. Topography compiled and drawn by O. B. Harden. Definition of gravels by H. C. Lewis. [From the Grand Atlas, Division 1, Part 1, Sheets 35, 36, and 37]

DATE  1881

SOURCE  Delaware County Institute of Science, Media, Pa.

SIZE OF ORIGINALS Each atlas sheet measures 26 inches wide by 32 inches high. The composite of sheets 35, 36 and 37 shown above was made from the three scans that were cropped to remove their borders and overlaid in Adobe Photoshop. If printed, this composite would measure 65″ x 30.5″.

WHY I LOVE THIS MAP  I’m not a geologist, and I have only basic understanding of the various formations presented on this map, but in general I love geological maps for their bright colors and their interesting details. I also have a subliminal attraction for any map that shows something that is out of sight and underground. My research into the sewers and historic landscape represents a similar kind of excavation, trying to get beneath the surface and reveal what once was.

MORE INFORMATION The Second Geological Survey of Pennsylvania was published in many small volumes, with all the associated maps folded up and tucked into pockets in the back of each book. As these maps aged, the paper on which they were printed yellowed at the folds and often became brittle as well. When such a map is scanned, the image becomes checkerboarded with rectangles edged with this discoloration.

The good news is that the state also published what is called the Grand Atlas, with most of the maps that had been folded into the smaller books included in larger portfolios, on heavier paper, and folded only once or twice at most, and sometimes not at all. They contain the same information as the folded maps, but the resulting images, when scanned, are much cleaner and more pleasing to look at—suitable for framing, if you were so inclined.

The Philadelphia map shown above was printed in three separate sections. None of these were folded, but when scanned I discovered that each one had a slightly different shade of red at the top. I have pieced together these sections into one image, and then edited each section for brightness and color to get them to match as well as I could. The vertical lines indicate the different sections, and you’ll notice that besides the slightly different colors, they do not line up perfectly. (As much as I strive for it, I also think that perfection can sometimes be overrated—or in this case, way too time-consuming to achieve.)   

SIMILAR MAPS  The Delaware County Institute of Science has a complete six-volume set of the Grand Atlas and has scanned the Delaware and Chester County maps from those portfolios, which I may feature in a future Map Lovers’ post.  To view all the publications in the Second Geological Survey (and others done before and after), which include text and maps along with many beautiful illustrations, visit this link at the Penn State University Libraries.

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