About 200 million years ago, geologists believe, most of what is now Baltimore, Carroll, Montgomery and Frederick counties was part of one high, craggy mountain range of crystalline rock.

Then the rains of 75 to 100 million years wore down the great mountains until they were piedmont, or foothills, and in the process washed billions of tons of gravel and sand toward the sea.

The earth buckled along a line dividing the sediment from the hard rock. The deposits of gravel and sand east of this "fall line" make up much of what is now Prince George's County, which today is a major source of the precious building materials.

Together the two products are "the most important resource that the country has, if not the only one," according to Dale Hutchinson, a member of the county planning department staff.

The 5.4 million tons mined in 1979 brought between $27 million and $32 million into Prince George's economy, not counting the value of finished products such as concrete and asphalt.

Most of the gritty gold is controlled by a handful of family businesses begun during the depression years with a few dump trucks and a lot of land nobody wanted. It is dirty, unglamorous work. The great earth cranes rip off the top of the land and sometimes never replace it. Lines of dusty dump trucks haul tons of rocks that occasionally fly off and break an automobile windshield.

Many Prince Georgians think they could do without the mixed blessings of such a natural resource. But the sediment rock constitutes 95 percent of the county's asphalt roads and 75 percent of a concrete block. The average house contains 50 to 100 tons of sand and gravel. One block of a subdivision street has 400 to 600 tons.

Importing the sand gravel for these purposes could be prohibitively expensive.

"You transport it 40 miles and you've doubled the price of the product," said Charles Hawkins of the Silver Spring-based National Sand and Gravel Association. "People don't like having it next to them. It's dirty and noisy; that's hard to refute. But these are natural resources. It builds our highways, our courthouses and schools. You couldn't build Metro without it."

Prince George's, which produces more sand and more gravel than any other county in the state, provides most of the materials used in the Washington metropolitan area.

The Prince George's county fathers have recognized the importance of sand and gravel in land-use planning for as long as they have been planning land use. They are specifically mentioned in the master plan as a valuable resource that should be exploited whenever and wherever possible. Yet, as large portions of the county have grown from rural backwaters to urban and suburban centers, the sand and gravel producers find themselves pressed by uneasy neighbors.

Planner Hutchinson said, "Sand and gravel mines are permitted virtually anywhere in the county by the special-exception (to zoning restrictions) process. That has created some problems."

One such problem arose recently in a subdivision called Gordons Corner, near Marlow Heights. The owners of the $85,000 to $150,000 houses there were aghast at the prospect of a sand and gravel mine on the 64 acres behind their tract. The proposed operation would have come within 30 feet of their backyard swimming pools. The Silver Hill Sand and Gravel Company's special zoning exception was approved by the county planning commission and the administrative hearing examiner, but was rejected last April by the County Council, which usually has the final say in zoning cases.

The council's ruling can be appealed to circuit court, although the Gordons Corner decision has not been challenged as yet. Two other council decisions against the industry have been reversed within the last 14 months. One reversal was the council's rejection of the Silver Hill company's plan to build a processing plant near the headwaters of the Zekiah Swamp, near Cedarville in southern Prince George's. Ecologists argued that runoff water from the plant would destroy the swamp's ecological balance. A judge ordered the County Council to approve construction of the plant; the council is expected to appeal that decision to the Maryland Court of Special Appeals.

Sand and gravel producers argue that their business is very much in harmony with the ecology of the land, and that too much regulation will make them an endangered species.

Sand and gravel deposits are usually within 30 feet of the surface. The "overburden'' of dirt is scooped off by a giant steamshovel with a bucket that holds five cubic yards of earth. The sand and gravel are forked from the ground by front-end loaders and put into waiting dump trailer-trucks or the new bottom dumpers -- sleek trailers with triangular beds and pneumatic doors that open from the bottom.

The sand is separated from the gravel at processing plants where conveyor belts resemble huge roller coasters. Mechanically operated shaking screens and water clean the gravel and sort it into three sizes.

The lion's share of the business in the county is done by five large producers: Contee Sand and Gravel, Buffalo Sand and Gravel, Brandywine Sand and Gravel, A. H. Smith, and Silver Hill Sand, Gravel and Concrete.

Samuel Bevard, 79, founder of Silver Hill, is fond of pointing out that the hills where the subdivisions of Hillcrest and Marlow Heights now stand were steep and unsuited for building until he removed millions of tons of gravel and graded the area during the 1940s and '50s. The Marlow Heights Shopping Center on Branch Avenue is approximately 20 feet below the level of the road because, "We took about 120,000 cubic yards out of there," Bevard said.

Bevard started his business in 1939 with eight trucks he brought down from his native Baltimore. His first digging was on leased land along Branch Avenue, where the Ourisman Chevrolet dealership now stands.

"It was four employes, a foreman and a crane operator," said Bevard, who gave up active management of the business about 10 years ago. "We were rubbing one nickel against the other than. I was the salesman and my brother was the office manager. We'd go out like anyone else and beat the bushes."

Today the companies that comprise Bevard Industries, including two processing and concrete manufacturing operations, have a fleet of 160 trucks, bulldozers, front-end loaders and other equipment. They employ 300 persons, and annual business is estimated conservatively at $16 million.

But in the '40s and '50s, when the Bevard business was growing slowly, buying rural land cheap and mining leased land, the major competitor was a District-based firm that dredged the lower Potomac River and brought sand and gravel by the bargeful to the docks near the South Capitol Street Bridge.

Then in 1968, federal environment regulations banned dredging in the Potomac, and the sand and gravel business boomed in Prince George's.

"When Potomac went out it was just the time that the subway was getting started," said Marion Bevard, 42, one of three Bevard sons now running the family enterprises. He estimates that Metro construction boosted sales by 50 percent.

An officer in county and state trade associations, Marion Bevard began working at age 10 and soon had operated every piece of equipment, from dump trucks to draglines. "They weren't as big as they are today, but they didn't have power steering then either," he recalled.

The associations exist largely to protect the industry from the encroachment of zoning and other forms of government regulation. Individually, the companies are represented by some of the best and most influential law firms in the county. They are also regular contributors to political campaigns, although the Bevards play down any resulting benefits.

No amount of help from associations, lawyers and politicians has been able to change the fact that continuing development, zoning restrictions and depletion of the deposits are making gravel and, to a lesser extent, and harder to find and mine. Many of the richest remaining deposits lie under and around population centers such as Gordons Corner.

"There's not that much left, close to Washington. It's all been covered over or mined out," said Karen Kuff, an economic geologist with the Maryland Geological Survey who has studied the problem of mining limitations. "In Prince George's the basic factors that have eliminated lands are government ownership, urban development and prior mining. I know that over 50 percent has been covered over, mined or preempted. . . . There's enough left in southern Prince George's to last quite a while . . . if it's all made available."

Indeed, the Bevards say their business is moving south, toward deposits in the still-rural, southernmost reaches of Prince George's and Charles counties.

"You can't stand still," said the elder Bevard. "You've got to keep going. I always say that hard work and good whiskey won't kill you."