Adirondack geologic history began more than a billion years ago -- perhaps hundreds of millions of years earlier -- when all of New York and the rest of eastern North America lay under the waters of a shallow sea that filled a long, narrow trough called a geosyncline. Into this primeval sea drained sediments from a land mass to the west and from an archipelago of volcanic islands to the east. No fish inhabited the waters; land animals would not evolve for another 700 million years.

As the millenia rolled on, the geosyncline, which stretched from Labrador to the Gulf of Mexico, became gorged with sand, clay, lava and volcanic ash. Eventually these deposits, solidifying into sedimentary rock buried deep into the sea floor, reached a thickness of possibly 15 miles. Then a collision of drifting continental land masses inflicted enormous pressures against the sides of the geosyncline, squeezing the sedimentary rock so hard that the upper layers were folded upwards to form mountains that may have risen 20,000 feet or higher. At the same time, the lower rock layers were compressed and contorted into metamorphic rock, reinforced by pockets of an igneous rock, anorthosite, formed by the cooling of magma squeezed up from deep within the earth's crust.

But no sooner had these newborn mountains thrust their summits skyward than the inevitable assualt by running water began to tear them down. Over a period of about half a billion years, erosion ground most of the rock into gravel, sand and clay that were swept down into a new geosyncline encompassing most of eastern North America. By roughly 510 million years ago, the primeval Adirondacks had been worn down to a beveled highland and the anorthosite was now exposed, having eroded much more slowly than the sedimentary rock that once surrounded it. The region was again flooded by shallow seas that left new layers of sediment and tiny marine fossils on the sea floor.

About 440 million years ago, the collision of the drifting North American and European continents began a new cycle of mountain building east of the Adirondacks. The first such activity, creating the ancient Taconic range, drove the sea eastward and made the eroded Adirondack highlands part of a wide coastal plain extending from Newfoundland to Virginia. Renewed subterranean pressures opened faults in the bedrock. More faulting and new intrusions of igneous rock followed the formation of the Acadian range about 400 million years ago and the Appalacians about 50 million years later. For hundreds of millions of years following this era of mountain building, the slow forces of erosion continued to whittle away at the Adirondacks, reducing them to gentle hills.

But about five million years ago -- relatively recently geologically -- the Adirondacks underwent a remarkable transformation. Increased pressure lifted the region like a huge dome, and new mountains began to grow along the old fault lines. Today all the major ranges of the Adirondacks are generally oriented in the southwest-northeast pattern of the fault lines, as are the major lakes, which rest in valleys between the ranges. As a result of this second uplift -- which continues today --some of America's oldest rocks are found in its youngest mountains.

Perhaps a million years ago came the climactic event of Adirondack history: the advent of the great ice sheets of the Pleistocene epoch. At least four times stupendous masses of ice invaded from the north, enveloping mountains, planing their peaks, grinding and polishing, scooping out lakes, stripping soil from Canada and depositing it in New York and New England, modifying the face of the region. Each time glaciers moved down with their massive tread, the land was depressed beneath their weight. Each time the ice receded, it left behind debris, forming drumlins (small hills) and moraines (ridges), damming valleys to create new lakes -- and, as the ice melted, yeilding new rivers and streams.

The last ice sheet left the final imprint on the Adirondacks. Bearing away the soil, the glaciers laid bare the underlying bedrock, flayed by debris imbedded in the brutally abrasive glacial mass, and steepened the walls of V-shaped valleys and widened their floors, creating new U-shaped configurations. Later, when the climate warmed some 10,000 years ago, the ice retreated and the Atlantic Ocean invaded the depressed Saint Lawrence and Champlain vallays, bringing with it beach sand and sea shells. Then as the earth's crust rebounded in response to the melting of its burden of ice, the sea withdrew.

The rebound continues today. From time to time residents of the Adirondack region are aware of tremors that momentarily rock their houses gently -- small earthquakes, never alarming, indicating simply that the earth's crust has not yet readjusted to the retreat of the last pressures of the ice. By the time such adjustments are made, a new set of circumstances -- perhaps another ice age -- will compel a further response from the environment.

And the landscape -- composed of the underpinnings of vanished ranges -- will repeatedly renew itself by crustal warping or upwelling of the earth's molten interior, and be destroyed repeatedly by the forces of erosion and gravity.