Portland Brownstone quarries, The

Chronicle of the Early American Industries Association, Inc., The, Sep 2002 by Guinness, Alison C

Introduction

All too often we fail to recognize the contributi,on of our geologic heritage to the progress of mankind on Earth. Today, we are dependent on fossil fuels, a geologic resource without which our civilization would not exist. Since our human beginnings, we have relied on rocks and minerals for our survival. We have used stone to leave monuments to ourselves and our progress. One such stone that played an important role in American culture was brownstone, an arkosic sandstone. During its heyday in the midnineteenth century, most of the brownstone used in the United States came from Portland, Connecticut, a small town lying on the east side of the big bend in the Connecticut River, across from Middletown, thirty miles north of Long Island Sound (Figure 1). All that remains of this once thriving industry are quiet quarry lakes surrounded by massive walls of stone, but Portland brownstone traveled to far away places and helped symbolize an era. Portland brownstone was so well known that Portland Stone, Brownstone, and Connecticut Stone became synonymous.

Portland brownstone was prominent in shaping the economy of the area, and it was a significant representative of architectural style and usage in the nineteenth century Brownstone not only crossed state economic lines, it also became a collective term for all buildings of a particular type. It is ironic that over a century after the "rage" ended and a great many brownstone structures had been demolished, we are only now beginning to appreciate the beauty, style, and majesty of those buildings.

Geologic Setting

During the early Mesozoic era, between 220-195 million years ago, the sandstones that became the Portland brownstone were deposited in a system of continental rift basins that developed along the East Coast of the United States during the embryonic phase of the opening of the Atlantic Ocean when the supercontinent Pangaea broke apart. The Connecticut rift valley is bordered to the east and west by large fault zones that formed a wedge-shaped depression that filled with the eroded remains of the crystalline rocks of the Eastern Highlands, forming alluvial fans on the valley floor.

The brownstone of the Portland quarries is part of the uppermost and youngest of the series of sedimentary units deposited in the Connecticut rift zone. This unit, called the Portland Formation, forms a layer nine hundred to fifteen hundred feet thick.1 In the Portland quarries, the brownstone is present in massive planar beds "2-18 feet in thickness, varying from 20- 100 feet in width and in length from 50-150 feet."2 Variations in the depositional geometry within the beds affected quarrying.

Coarse-grained sands consisting mostly of quartz, mica, and feldspar form the Portland brownstone. Because of the high feldspar content in these sediments, they are classified as arkose, which ranges in color from light pink to red to brown to purple, giving rise to such distinctive names as "redbeds" and "brownstone."3 The sediments were compressed and cemented together over time with a moderately hard albite cement.4 The coloring of the sandstone is a result of oxidation of ferric oxides from limonite to hematite, giving the Portland brownstone its distinctive, deep brown color.5

Passing animal life, such as dinosaurs and other reptiles, left footprints and track ways in the deposits. Many slabs containing footprints could not be saved in the day-to-day operation of the quarries. One piece of brownstone was found in 1874. It was sixty feet long and contained a track way of twenty-one footprints. It was of such significance that during that summer's meeting of the American Association for the Advancement of Science in Hartford, several geologists took the time to go to the quarries to see them.6 Among the most common of the Portland tracks are those of Anchisauripus, Grallator, Otozoum, Eubrontes, and Batrachopus.

The sediments deposited in the wedge-shaped rift were tilted ten to fifteen degrees east. Gentle folding and warping took place contemporaneously and were followed by faulting that broke the rock units into large blocks. During the Cenozoic, tectonic activity affecting the quarry area included more faulting and warping. This, and the earlier tilting, left the brownstone with nearly horizontal bedding and created a system of joints or cracks perpendicular to one another that expedited the quarrying process. Quarrymen would use these joints and the bedding to loosen massive blocks of brownstone.

As quarrying was carried on deeper and deeper into the bedrock, strain was encountered in the rock. Strain release was accomplished by spontaneous movement of the block of stone being excavated. Johnston described the phenomenon as having first occurred around 1851, at a depth of 120 feet below the surface where a beginning was to be made in the stratum then constituting the bottom of the cavity that had formed. For this purpose the workmen were directed to cut with their picks a channel or groove, a foot or more wide, quite through the stratum, and some fifty or eighty feet in length. The stratum was found to be about six or seven feet in thickness, and it happened that the channel or groove was cut in an east and west direction. When the channel had been sunk to within about nine or twelve inches to the bottom of the stratum, suddenly the remaining stone at the bottom was crushed to fragments with a loud report by an enormous lateral pressure, and the men in alarm leaped from the excavation they had made. By examination, it was found that the walls of the groove or channel just made had approached each other about three-fourths of an inch, the stratum through which they had cut having on one side sudden this distance in its ancient bed.7