Stop 2: Legg Mason Tower - 100 Light Street
From the corner of Calvert and Lombard Streets, walk west (left) along Lombard Street for one block to Light Street. The Legg Mason Tower is on the west side of Light Street.
Two types of stone are the focus of this stop: granite and travertine. The exterior plaza and the low perimeter railings that surround the plaza are made of pink granite, with a flamed finish. Notice also that the pillars and upper exterior of the building is faced with the same granite (Figure 2a). Observe the color and crystal size of each mineral.
The granite, quarried in Alicante, Spain, was formed during the Precambrian Era over 544 million years ago. This area of Spain has 20 to 30 quarries and is known for the great variety of pink granites (as well as red and black marbles typically used in building interiors) (source: Hilgartner Natural Stone Company). It is possible for two different-colored granites to come from the same quarry or nearby quarries. The difference in color can be because of changes in the relative amounts of each mineral present in the rock.
The color of a mineral, however, is not always constant. Many minerals, such as quartz and feldspar, vary in color. The different colors can be due to variations in a mineral’s chemistry, defects in crystal structure, or the presence of an impurity. For example, iron oxide can be an impurity that gives a reddish color to many minerals including some feldspars and calcite.
The two exterior side walls of the building (at plaza level) are faced with the second stone of interest: travertine, a sedimentary rock (Figure 2b). Travertine is a type of freshwater limestone. It is composed of the minerals calcite or aragonite (two forms of calcium carbonate) which formed in warm or hot springs, rivers, lakes or caves, by the precipitation of these dissolved minerals out of solution. This means that the stone is made of minerals that were deposited (crystallized) when the characteristics of the water changed, often because of evaporation or agitation of the water and often in combination with organic growth (commonly algae or bacteria). Stalactites and stalagmites are types of travertine formed in caves.
Italy is a well-known source of travertine. Travertine near Tivoli, Italy, (slightly east of Rome) formed in ponds and lakes associated with warm springs. These deposits of travertine are extensive and have been quarried since Roman times for use as building and ornamental stone in many structures, including parts of the Roman Colosseum (Folk, and others, 1985). Some warm springs are active in the same region today, and travertine is still being deposited.
The travertine at the Legg Mason Tower originated in the Middle East (source: Hilgartner Natural Stone Company). As each layer of calcium carbonate covers the previous layer, the rock may take on a banded or layered appearance. Layers may be thin and flat or rounded. Travertine is typically white, tan and/or cream in color. The reddish brown color of the travertine at the Legg Mason Tower is likely to be due to the presence of some iron oxide in the limestone. Some of the darker bands and irregular voids appear to be related to plant-influenced precipitation, perhaps once incrusting algal or bacterial colonies. White rectangular fragments appear to be deposits that were broken and moved before the sediments were completely hardened into rock.
Limestones can be vulnerable to weathering particularly from acids in rain and the air which over time can gradually dissolve, discolor or disintegrate the stone. The travertine at the Legg Mason Tower has a protective coating to help protect the finish from such weathering (source: Hilgartner Natural Stone Company).