"Daddy," I hope my children will have the presence of mind to ask one day, "where does the West begin?" I will take them into the front yard of our house in Austin, point down the street to where a high limestone terrace will be visible in the distance through the branches of winter trees, and say, with sober authority, "There."

The Balcones Escarpment. It is geology's most fateful mark upon the surface of Texas, a bulwark of cracked and weathered rock that extends in a pronounced arc from Waco to Del Rio. It is the Balcones that creates the Hill Country, that sets the stage for the Edwards Plateau and the High Plains beyond. The cotton economy, for our schematic purposes, ends at the base of the escarpment, where the rich blackland prairie that sustained the courtly reveries of the old South runs literally into a wall. Above that mass of limestone there is only a veneer of soil, and the country is hard, craggy, and scenic--cowboy country. The distinction is that sharp: farmers to the east, ranchers to the west.

But of course the escarpment was there long before human beings came and parceled themselves out along its boundaries. We can trace its ancestry back 300 million years, when some primitive version of the North American continent collided with another and still-unknown landmass. For eons these continents, riding forward on their crustal plates, ground into one another with such inexpressible force that they left the landscape rumpled with mountain ranges. One such legacy was the Ouachita Mountains. Today the Ouachitas occur primarily in Oklahoma and Arkansas, but there was a time when they extended southward in a graceful curve across the face of what is now Texas. In the long run they were diminished by erosion and pressed downward by the weight of newly formed sedimentary rock in the Gulf Coast Basin. Charles Woodruff, a consulting geologist in Austin, compares the subsided Ouachitas to a leaf on a drop-leaf table, slowly bending at the hinges beneath the weight of a Christmas turkey and platters of sweet potatoes and pickled peaches. Buried over time beneath thousands of feet of rock, the Ouachita hinge still retained enough flexion to send periodic tremors through the earth.

It was during the Miocene era, between 5 and 20 million years ago, that the fault zone in the buried Ouachitas created the Balcones Escarpment. It happened slowly, earthquake by earthquake, in the salad days of three toed horses and giraffelike camels, while the whole continent stretched and seethed in its epochal growing pains. The west side of the Ouachita hinge was hitched up, while on the east the land subsided still further.

The escarpment served as a trap for the weather masses that moved in from the Gulf, and as a result the rainfall along the fault zone was intense, eroding the soil along the new stream channels and declivities it carved in the bedrock. The stark limestone that made up the escarpment could support only thin, limy soils, but east of the fault the ground remained a fertile compost.

Rainwater poured into the fractured rock along the fault zone, creating a vast aquifer that fed artesian wells and glorious spring discharges. Here, at the base of the escarpment was the place to be. A settler could stand with his feet in waxy black soil, listening to the sound of groundwater, and look up at the Hill Country, filled with game and scenery. It was along this line that the Spanish built their first missions. (The rising tiers of limestone to the west reminded them of balconies, and that is how the escarpment got its name.)

Now, of course, a chain of cities parallels the Balcones. Between Austin and San Antonio the fault zone runs more or less even with Interstate 35. It was along this fault, according to historian Walter Prescott Webb, that "the ways of life and living changed. Practically every institution that was carried across it was either broken and remade or else greatly altered."

In our time the line between east and west is not as absolute as it once was. Thanks to irrigation, cotton grows on the High Plains, and the steep upthrust hills that once stopped prudent westward migrants in their tracks cause barely a whisper in an automatic transmisison. But the fact remains that when we climb to the top of the Balcones Escaprment, we cannot help thinking of ourselves, at least for that moment, as Westerners. Such a self-image may be an illusion, but the boundary that creates it is as solid as rock.