Everybody thinks of Philadelphia is a city of brick architecture but in many of the outlying areas, it really stone architecture dominates. Thomas Wertenbaker, in his book, The Founding of American Civilization the Middle Colonies, observed that if you take an imaginary line from Trenton to Wilmington, much of the historical architecture on the east side of this line is made of brick and much on the west side is made of stone. Geology plays some role in this, since the area on the east side of this line is close to the Delaware where there are plentiful clay deposits and excellent building stone can be found very close to the surface on the west side of this line. Another factor is who settled in the areas. English settlers, with a strong tradition of brick construction, often settled along the Delaware River, and many people from central Europe, with a tradition of stone construction, settled on the west side of this line like the German settlements along the Wissahickon.

Several of the local stones are very suitable for building construction. In the northwest area of the city, along the Wissahickon valley, people found beautiful schist with mica flakes which was soft and easily quarried. In Bucks County, and the areas around Trenton, a red sandstone was available. Along the Schuylkill River, north near Valley Forge, a white marble with blue veining was found and used for both steps and fireplace surrounds, and even occasionally whole buildings such as the Second Bank. In the Victorian era, green sandstone from Chester County, called serpentine stone, was used but was very vulnerable to water deterioration giving it the dubious distinction of being one of the least satisfactory building materials ever used.
Stone
Molland House, Horsham
Stone masonry can be categorized by its finish into either rubble or ashlar. Rubble masonry uses stone more or less as it comes out of the ground but often dressed on the front face to form a flat wall. The mason may trim or dress other faces of the stone to fit the stone into the wall easily but rubble masonry usually ends up having very wide mortar joints to compensate for the differences in fit between one stone and another. Ashlar masonry is any stone that has dressed square so that it can be installed in the wall with smaller joints. In some cases ashlar stones are dressed to all their same size making the horizontal joints continuous and the vertical joints appearing at regular intervals which creates a more formal appearance. Included in this category are buildings with substantial stone fronts like the First Bank of the United States, even though the rest of the building is brick.