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|A Geologic Walking Tour of Building Stones of Downtown Baltimore, Maryland||contact: Dale Shelton (email@example.com)|
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STOP 1: THE GALLERY - 200 East Pratt Street
The Gallery (Figure 1a) is a shopping area located across the street from the Inner Harbor. The Gallery shopping area has interior floor tiles made of limestone, a sedimentary rock (Figure 1b).
The tan limestone is from central France. The limestone formed from sediments and shells deposited in shallow, clear seawater millions of years ago. As layers of sediment slowly built up over time, the burial pressure and natural chemical cements changed the sediments into solid rock. The marine fossils present and the fine grain size of the material surrounding the fossils reflect the environment of deposition. Not all of these fossils are easily recognized because of the way the stone was cut. Most fossils have been sectioned at various angles between horizontal and vertical. The small, black tiles on the floor (Figure 1b) are also limestone. This limestone came from Spain.
Exit The Gallery, turn right and walk north along Calvert Street, where a polished red granite covers the exterior walls of The Gallery (Figure 1c). This red granite is from Taivassalo, Finland, and is of Precambrian age, more than 544 million years old. Granite is an igneous rock formed by the cooling of molten magma. The slower the magma cools, the larger the crystals, and conversely, the faster it cools, the smaller the crystals. The large crystal size indicates that this granite formed deep inside the earth where cooling took place slowly. The two minerals essential to any granite are feldspar and quartz, but mica is another mineral often present. The feldspar crystals in this rock are pink to red in color. Feldspar minerals are the most abundant mineral group in the earths crust. The smoky colored mineral is quartz, the second most common mineral found in rocks. The black mineral is biotite, a type of mica. One distinct property of micas is that they split into fine sheets or layers along cleavage surfaces similar to pages of a book.
Further along the building wall, about half way along the block, there is a section of the granite which has a smooth, polished finish adjacent to one with a rough texture, or flamed finish (Figure 1c). A flamed finish is caused by exposing the surface of the rock to the heat of a torch. The differential temperature causes rapid expansion and flaking of surface fragments. Different finishes provide variety in the texture and appearance of the same stone.
Continue walking north along Calvert Street. At the corner of this block, within the same office-shopping complex, look to the right at the interior of 111 South Calvert Street. The same pink to red granite from Finland is used in the lobby floor along with inlaid strips of South Dakota gneiss. The gneiss formed during the same Precambrian Era as the granite. This is a good opportunity to compare the igneous granite with the gneiss, which is a metamorphic rock. The granite has conspicuous crystals of the quartz, feldspar, and mica, scattered throughout. In the gneiss, the same components tend to occur in alternating irregular bands of lighter minerals and darker, generally elongate, minerals that were recrystallized under tremendous pressure (Figure 1d).
|Figure 1a: The
Gallery entrance (Stop 1).
Figure1b: Fossiliferous limestone tiles on the lobby floor (Stop 1).
Figure 1c: Granite on the outside wall of The Gallery. Rough “flamed” texture left; polished surface right (Stop 1).
Figure 1d: Pink granite (lower right) and gneiss (upper left) on the lobby floor of 111 South Calvert Street (Stop 1).
This pamphlet was prepared by
Sherry McCann-Murray, with contributions and photography by the Environmental
Geology and Mineral Resources Program of the Maryland Geological Survey.
Adapted for the Internet from Educational Series No. 10. For more information see Building Stones of Maryland .
Compiled by the Maryland Geological Survey, 2300 St. Paul Street, Baltimore, MD 21218
This electronic version of "A Brief Description of the Geology of Maryland " was prepared by Bob Conkwright, Division of Coastal and Estuarine Geology, Maryland Geological Survey. Please send comments on this page to Dale Shelton (firstname.lastname@example.org)
State of Maryland
Department of Natural Resources, Resource Assessment Service