Deposits of reddish-brown sandstone lie in long, narrow bands along the eastern seaboard from Nova Scotia to North Carolina.

In the band of sandstone-bearing rock known as the Hartford Basin, settlers of the Connecticut River Valley used sandstone boulders weathered from the Portland Formation for building as early as1650. Actual quarrying of Portland Brownstone began in 1725, and reached its peak in the mid- to late 1800's.

The 1880 federal census tally of NYC buildings, reported that 78.6% of stone buildongs used brownstone, in whole or in part. The prevalence of Portland sandstone for construction during this era lead to the term "brownstone" being synonymous with "rowhouse" in the northeastern US.

Headstone at Trinity Church
(Mid-1700's)

Most brownstone was quarried in Portland (CT), where individual, near-horizontal beds are approximately 0.7 to 6 meters thick, and have a total thickness of 1000 meters.

Smaller quarrying operations existed in Longmeadow (MA), Newark (NJ), and Hummelstown (PA), but these deposits were smaller, and consisted of sandstones of variable color and quality in steeply dipping beds that were difficult to quarry. The color variability of the Newark Brownstone is well displayed in Lower Manhattan's Trinity Church.

Newark Brownstone in Trinity Church (1839-1846)
Broadway at head of Wall Street

The Portland Brownstone is a coarse sandstone (feldspathic arenite), with feldspar content as high as 65%. Quartz and mica comprise most of the remainder of the rock. These detrital components were eroded from Ordovician gneisses and schists of Connecticut's adjacent Eastern Highlands. Albite cements the sand grains.

The red color is due to iron oxides. Freshly quarried sandstone tends to be yellow-brown due to the presence of limonte. Dehydration of the quarried rock due to extended exposure to the air, results in conversion of limonite (FeO[OH]) to hematite (Fe2O3). The distinctive red-brown color of brownstone is due to the presence of hematite.

The common problem of spalling may have been due to hasty construction rather than an inherent fault with the rock itself. Rock from Portland was quarried from depths below the water table. Therefore, water filled the stone's pores. Initially, stone quarried from Portland was allowed to "season"—that is it was left to dry out gradually. During the housing boom of the late-1800's the demand for Portland Brownstone exceeded the supply.

Accordng to Mike Meehan, a geologist who re-opened a Portland Brownstone quarry in 1993, "Face-bedded stone, still full of water, was routinely applied to many of the 50,000 row houses built during the late 1800's. If it was done late in the season, just as freezing set in, the stone's disintegration was assured."

Naturally bedded stone, is that in which the rock is cut perpendicular to the bedding. This type of application exposes many beds within an individual slab, possibly exposing variations in grain size, color and texture. It is also more difficult to cut, and therefore more expensive. However, naturally bedded stone is more resistant to weathering.

ENVIRONMENT OF FORMATION

In the Late Triassic time (~ 220 million years ago) to Early Jurassic, North America began to rift away from the rest of Pangea. A series of normal faults broke the mountainous region into a series of elongate ridges and valleys. As the valley floors dropped, rivers swept sediments eroded from adjacent highlands into the lowlands where large lakes developed during periods when the climate was wet.

During extended dry periods, the fault-bounded lakes shrank and stream channels migrated across the valley forming broad alluvial plains. Wedges of sand and coarser sediments accumulated in alluvial fans adjacent to the valley's border faults, and later lithified to form coarse sandstones and conglomerates. Coarser sediments decrease towards the center of the basins, whereas beds of fine sand, silt and mud become more abundant.