On the Potomac Heritage Trail -- upstream of Roosevelt Island on the Virginia side of the Potomac -- there is a large chunk of iron boiler, apparently from many years past. What was the function of the steam engine?-- Hank Bruhl, Annandale

If you walk along the banks of the Potomac River near Donaldson Run in Arlington County -- as Answer Man did one morning last week -- you will hear the near-constant roar of jet airplanes landing at Reagan National Airport.

Eighty years ago, you would have heard something else entirely: explosions. In fact, Answer Man suspects it probably would not have been a good idea to stroll there because, from about 1794 to 1938, various stretches of the Potomac between Pimmit Run and Spout Run were active stone quarries. Beware falling rocks, indeed.

The quirks of geology make this area a particularly convenient place to get types of stone known as mica schist and granite gneiss. The hard metamorphic rock on the Virginia side of the Potomac resisted erosion and so was arrayed in cliffs. Stoneworkers didn't have to dig down to get at the rock; they could just dig across.

That wasn't necessarily easy, but humans are endlessly resourceful, and quarry owners figured out a way to cleave stone from the bluff. Boilers were floated on the river and set up on barges or on the riverbank. Steam was used to run drills, which bit into the rock and created holes about the diameter of a silver dollar. Dynamite or blasting powder went into the holes, along with fuses. A worker would light the fuse, shout "Fire in the hole!" and then run for cover -- along with everyone else.

Gilbert Vanderwerken owned a quarry on the Potomac from 1867 till his death in 1894. His grandson Charles Grunwellused to visit the site when he was a boy. Charles wrote about the experience in a 1966 article in the Washington Star: "The blast that shortly followed shook the earth and was heard for miles around. Hundreds of tons of rock were torn from the cliff and hurled rumbling down into the quarry yard."

Quarry workers also used hand drills and sledgehammers to harvest the stone. They earned $1.25 a day.

Most of the stone had a bluish color, prompting the nickname "Potomac blue stone." Larger pieces were used to build Georgetown University's Healey Building, St. Elizabeths Hospital and the sea wall at Hains Point. Rubble was crushed and used for street foundations.

The last company to chip away at the palisades was Smoot Sand & Gravel Corp. Quarry operations ceased in 1938 after the cliff had been eaten back so much that it was no longer economical to transport stone to the river. Workers who toiled at the quarry continued to live nearby, though, in shacks that came to be known as "Little Sicily." In 1956, the last of them -- three Italian quarry men ages 70, 72 and 80 -- were ordered to move to make way for the George Washington Memorial Parkway.

Traces of their equipment remain, however: three big rusted metal tanks -- their seams held together with enormous rivets -- and a concrete box once used to store explosives. There's another boiler stuck in the Potomac near Gulf Branch, visible when the river is low. Some of the stone still bears the holes made by the steam drills.

To see for yourself, go to Potomac Overlook Park and follow Donaldson Run to the Potomac and turn right on the Potomac Heritage Trail. It's about a 1 1/2 -mile hike downriver -- and back in time.

Julia Feldmeier helped research this column. Send your questions to answerman@washpost.com.