Memorial Hall — Local services in Cambridge

Common uses of slate are on old school blackboards, pool tables, and shingles on older buildings. We are going to learn about the latter with this earthcache at Memorial Hall on the campus of Harvard University.

To get credit for the earthcache, send the answers to the questions below to my online account.

Harvard's premier slate-roofed structure, Memorial Hall (1878), displays rock from the two most important slate-producing regions of New England. The red shingles came from quarries near Granville, New York, the green shingles from nearby Fair Haven, Vermont, and the black from Monson, Maine. They are also some of the oldest rocks on campus, ranging in age from the pea-soup-green, 550-million-year-old Mettawee Slate, through the red Indian River Slate, to the black Monson, a 400-million-year-old unit, known to geologists as the Carrabassett Formation.

The amount of oxygen and the type of sediment in the depositional environment determined the colors of the Memorial Hall roofing slate. The red in the Granville slate resulted from the erosion of iron-rich soils that were exposed to an oxygen-rich environment. Granville has the only red slate quarries in the United States. If the iron oxides were deposited in an anaerobic environment, they produced a green- to purple-colored sediment. The black slate also formed in an oxygen-starved environment, but contains abundant organic matter that imparted its black coloration.

The formation of Memorial Hall's roofing slates started with clay and silt washing off the North American continent into a vast sea, the Iapetus Ocean (Iapetus fathered Atlas, for whom the Atlantic is named). For 150 million years, the Iapetus served as a dumping ground for sediments that sank slowly into the deep waters of the ocean, building up several thousand feet of extremely fine-grained layers of shale.

During the entire depositional history of these sediments, the Iapetus was slowly closing. As the ocean shrank, it transported a volcanic island arc (similar to Japan) that had risen in the middle of the ocean toward a collision with North America. When the arc slammed into the continent, it pushed the marine-deposited Vermont and New York sediments up onto the land, simultaneously burying and folding them. The weight of this mass of land compressed the sedimentary beds, metamorphosing the shale into a slate. The stress of the collision also caused the grains of clay, which is a flat, sheet-like mineral, to align themselves into a rigidly parallel arrangement, like organizing a collapsed house of cards into a tight deck. The Monson slate metamorphosed in a similar process 50 million years later.

This alignment of clay minerals, which facilitates the even and clean splitting of slate into distinct layers, has made slate an important roofing material for hundreds of years. The same qualities make it useful for floor tile, billiard tables, and blackboards. Immigrants from Wales played a critical role in establishing the slate industry in America. They were the first to recognize the high quality of the Monson slate deposits in the early 1800s and also helped to expand the quarries in Vermont and New York in the 1850s.

Sunlight has faded, but not weakened, the slates of Memorial Hall, put up in the 1870s. Slates varying in color from similar ones around them are replacements. The soaring central tower was clad in patterned slate until 1897, when it acquired clocks and copper sheathing. The tower burned in 1956 and had not been replaced until August 1999, that it received its final facelift.

Not all of the original Memorial Hall roofing slate is intact. In two restoration projects, workers replaced shingles that had broken because people walked on them, especially at the base of the roof, and other shingles that had cracked because of deterioration of their iron fasteners. Overall, though, the slate has proven...

A great building to check out the details on from the stained glass, to the carved figures that circle the roof. Harvard built memorial hall in memory of students lost in the Civil War. The building itself is housing for students...

Beautiful place you...