Mile 127.4 - The commemorative signboard reads:

The Round Top Hydraulic Cement Company operated at this site from 1863 to 1909, producing 2,200 barrels of hydraulic cement per week.  By the 1880's it was one of Washington County's most flourishing businesses.

 In 1838, George Shafer was authorized by the canal company to build a cement mill at the foot of Round Top Hill.  Shafer rented the mill, five stone quarry tunnels and the necessary water power to successfully produce 200,000 bushels of cement over a period of 25 years. In 1863, the mill was purchased by partners Robert Bridges and Charles William Henderson and renamed the "Round Top Hydraulic Cement Company".  A 16-foot diameter overshot waterwheel operated 4 pairs of grind stones, each 5 feet in diameter.  There were 8 supporting cement kilns, each fired by coal which was shipped by canal boat.  The mill employed up to 100 workers who packed the cement in 300-pound barrels, and 50 and 100 pound sacks.  It was then shipped by canal boat or sent across the Potomac River by cable for transport on the Baltimore and Ohio Railroad.

By the end of the 19th century, the use of natural cement declined in favor of slower setting and stronger Portland Cement.  In 1909, the Round Top Hydraulic Cement Company closed.

Read about the bats that hibernate in the Round Top Mines.

Detail of two of the kilns.  There are eight.

The remains of the mill. As I understand how the process worked, they stacked stone and coal in the kilns and burned the stones into quicklime. The mill would then be used to crushed the processed stones into cement.