DRIVE EAST FROM ANNAPOLIS OVER THE CHESAPEAKE BAY BRIDGE, and you'll be crossing the country's largest estuary. In a few miles you'll also be crossing over the graves of several ancient estuaries, the long-buried ancestors of today's Chesapeake.

The view approaching the bridge is dramatic: a curving roadway and two looming towers with long, down-sweeping suspension cables. But the view from the bridge is spectacular: it offers a rare, high-angle vista of the Bay. To the north, you'll see the Sandy Point Shoal Lighthouse along the Western Shore. To the south you'll catch a glimpse of the crumbling pier for the old ferryboat landing and, farther out, you'll probably see several coal ships sitting at anchor, waiting to head up to Baltimore. On a summer or fall afternoon, you may see a jumble of distant sailboats, tiny white flags working out from Annapolis, setting up for a race.

As you approach the high point of the bridge, the roadway curves gracefully to the east, aligning the center span at a 90-degree angle to the shipping channel below. If you look dead south from the top, you'll see the buoys outlining the shipping channel, and if your timing is right you may see a bulk carrier or container ship from the far side of the world aiming its long metal mass straight under your car. This is the highest part of the bridge, but this is not the deepest water you'll be crossing.

Once past the towers, you start the long, straight downhill schuss toward Kent Island, the first chunk of the Eastern Shore of Maryland. Nearing the far shore, you pass through a shorter, cage-like bridgeworks, a cantilevered span that stretches across a second channel. Seemingly an afterthought added to let workboats and tugboats pass through, this smaller span crosses the deepest water under the bridge.

This is where your trip into the past begins. The water is deepest near the far shore because 10,000 years ago the old channel of the Susquehanna River ran through this spot, back when the current Chesapeake Bay began taking shape. In that era, Kent Island was the high point of a high ridge that loomed above an ancient Susquehanna River valley. All those Eastern Shore islands stretching to the south — Poplar, Tilghman, Taylor, Hooper's, Smith and Tangier — were ramparts above the same river valley.

At the end of the last ice age, that old river was flooded by rising sea levels and turned into an estuary, a zone where freshwater and seawater meet and mix. Estuaries are sediment traps, and over the centuries those sediments finally filled in and buried the old axial channel. The scientists who discovered this buried paleochannel call it the Cape Charles Channel because it ran all the way down the Bay and out past the southern capes at the mouth of the Chesapeake.

Kent Island is the second stage of your magical history tour. As soon as you come off the bridge onto the island, you are driving over the grave of a second channel for an even earlier Chesapeake. Buried below this part of Kent Island is a paleochannel that formed farther to the east during an earlier ice age. Some 150,000 years ago, the Susquehanna River ran under this land. That earlier river also became an earlier Chesapeake. It also filled up, and the island now covers it.

Several miles farther east, you'll cross over Cox Creek and start passing gas stations and shopping malls, all of them built atop the gravesite of a third buried paleochannel. Some 300,000 years ago or more, a much earlier Susquehanna River ran here, became an estuary, filled up, and became part of an island.

Those buried channels were uncovered by three geologists: Jeff Halka of the Maryland Geological Survey, Steve Colman of the U.S. Geological Survey, and Carl Hobbs of the Virginia Institute of Marine Science. By uncovering these buried channels, they outlined in sharper detail the geologic forces that created the shape, size, and location of the current Chesapeake.

Those larger forces include ice ages crossing over to global warmings, and global warmings crossing back over to ice ages. During the heat, the lower reaches of many rivers along the East Coast became estuaries, and in this region the Susquehanna became the Chesapeake Bay. During the ice, estuaries became rivers. These crossovers happened several times over the last half million years.

The last crossover from river to estuary gave us the current Chesapeake Bay, the first time this estuary, for better or worse, has had humans living along its shores. During the end of the last ice age, according to one hotly debated theory, humans may have first trekked across the exposed Bering land bridge to begin the long, slow settling of North America. We are now well into that long-term interglacial, far enough along that humans known as scientists have been able to figure out some of the powerful forces that shaped the current estuary. These scientists built a bridge to the past, crossed over the bridge carrying the tools of geological science and brought back stories about the long and winding road that led to the birth of Chesapeake Bay.

With this issue of Chesapeake Quarterly, a magazine that began 10 years ago, we're bringing you two stories about the origins of a Bay that began 10,000 years ago. "Channeling the Chesapeake" looks back half a million years to the buried channels of earlier Chesapeakes; "Imprint of an Impact" looks back even further, recounting Wylie Poag's discovery of a big bang that shook the earth and ocean some 35 million years ago on the spot where the southern half of the Bay would later arise.

— Michael W. Fincham