At Great Salt Lake, some 4200 feet above sea level in the state of Utah, the lake today, 75 miles long and 50 miles across at its widest point, is only a remnant of what was here once -a gigantic inland body of water some 20 times as large, 67 times as deep, holding many hundreds of times the present volume of water.

Between 1873 and 1940 the water level dropped about 16 feet and the shoreline receded as much as ten miles in some stretches. Nearly 800 square miles of new land were thus bared, mostly malodorous mud flats or humpy chunks of land interlaced with ponds-a no man's land, barren, treeless, difficult to traverse, though at a few places a car can get through to the shoreline.

The Great Salt Lake, then, is a lonesome, harsh, even hostile setting today. Salt Lake City is 15 miles from the present shoreline. There are no summer cabins by the lake. There is no fishing because there are no fish. Only a few organisms live in the lake's dense brine: some species of algae and protozoans, a species of tiny shrimp and two species of fly.

In the 1890's there was a resort, a great big glorious pavilion with towers, built on a pier that ran a quarter-mile into the lake. A steam-driven train took people to the end of the pier. But this structure eventually found itself standing high and dry-deserted, partly destroyed by fire, a quarter-mile inland from the waterline.

Great Salt LakeThe remarkable shrinking of Great Salt Lake-from the middle of the 19th until the middle of the 20th cen­tury-resulted from an evaporation rate higher than the rate of flow into the basin from the mountain streams and rivers that feed it.

In the 1940s, as the bal­ance shifted once more, the water coming in, the inflow, began to exceed the evaporation loss, and as a result the lake began rising. By 1950 the water level had climbed to within 12 feet of its mid-19th-century level.

And then it began declining again. By the mid-1960s the water level has dropped nine feet off the 1950 mark.

The salinity-or concentration of salts-changes ac­cording to the lake's water level. In 1950, for instance, the salinity was about 25 percent-or some seven times saltier than the ocean. At times when the water level is lower, salinity often exceeds the saturation point, and the salts begin to precipitate.

Salts are brought into the lake by all the rivers and streams that feed it. Their water contains tiny amounts of dissolved salts and minerals washed from the rock and soil of the surrounding terrain. Since the lake has no outlet, the water can escape only by evaporation. And as water evaporates, it leaves behind its dis­solved salts and minerals. Here, then, are concentrated the salts drawn from Utah's mountains-in a kind of pocket ocean in a mountain basin.

A swim in Great Salt Lake is an interesting experi­ence. Like the Dead Sea, the water is so salty that you can't sink. You float with head, feet and arms buoyed up out of the water. When you come out, the water dries almost instantly. The salt remains, forming crys­tals on your skin.

The drying-out of this huge, ancient lake, which once covered the entire basin, has left an enormous desert on the western and southern sides of the present Great Salt Lake. This desert reaches 160 miles from north to south and some 70 miles to the west-a broad, grayish-white expanse strewn with billions of tons of salt, perhaps the most implacable environment in North America.

To attempt to traverse the Great Salt Lake Desert on foot would be to flirt with death. Although a highway crosses this barren plain, one drives it in the knowledge that for 50 miles on either side there are no other roads, no houses.

About 100 miles west of Salt Lake City the highway passes beside the Bonneville Salt Flats, an area 14 miles long and seven miles wide of smooth, white crust that resembles a sheet of ice over a lake. Of course, the "ice" is salt, and on this surface world land speed rec­ords are regularly established and broken.

From an airplane you can see the long, straight black line that serves as a guideline for the speed runs. There is no grandstand; there are no buildings; there is only the white plain and the adjacent naked mountains, a land­scape of aridity and emptiness.

How did this salt flat form so many miles away from the present shore of the lake? A depression existed at this spot in the ancient lake bed. As the big lake re­ceded, a super-salty solution was left behind in the depression. When this dried, only the salt remained- hard, level and smooth.

A few feet below the surface the Bonneville Salt Flats remain saturated with water. Drawn upward by wick action, this moisture acts to keep the salt firm. The moisture also keeps the salt cool. A cement pave­ment in that desert sun soon gets hot enough to fry eggs. But the salt usually stays ten degrees cooler than the air.

During droughts, the water under the salt dries up in spots. As a result the surface becomes crumbly, and potholes form. But in winter the flats are normally flooded by runoff from the nearby high ground, and the salt surface softens and becomes smoother.

How do we know that an ancient lake covered the entire region of the Great Salt Lake Desert? The evi­dence lies in the old high-water marks on the moun­tainsides. They are easy to discern. The highest lies 1000 feet above the present lake level. What draws the eye is that the marks are strictly horizontal. Experts have identified more than 20 of the former shorelines. Each gives evidence of the lake once having been at that level for a few hundred or thousand years.

This immense lake of the Ice Ages-called Lake Bonneville by geologists-was fed by the meltwater of ice caps that existed on the surrounding mountains. At its maximum, probably some 20,000 years ago, Lake Bonneville spilled over to the northwest in a mighty waterway that followed the course of the present-day Snake-Columbia river system to the Pacific Ocean.

Currently the great excitement around the lake con­cerns not the whys and wherefores of its geological history, but the legacy of that past in the form of min­erals that can be mined today. The water of the Great Salt Lake is about seven times as rich in minerals as sea water. Geologists estimate that some six billion tons of salt are held in solution by the lake waters. Besides common salt, gypsum and potash, the lake contains various compounds of boron, lithium, sulfur, magnesium and chlorine.

The most important of the lake's minerals commer­cially are its magnesium salts. Another major resource present in the brine is lithium. This soft substance, the lightest of all metals, is used in the manufacture of a diversity of products, including lubricants, ceramics and rocket fuels. In addition, medical scientists have experimented with lithium compounds in the treatment of mental disorders.

To extract this wealth, developers follow a proce­dure similar to the natural process of evaporation that causes the salts to accumulate in the lake. First, a series of shallow evaporation ponds are excavated and brine is pumped into them. The Sun and desert air take out the water, and the salt forms in deposits on the bottom. These are scooped out and refined. The process is an­cient. What is new is that modern engineers have learned to control the process so that all the various salts contained in solution-including the rarest-drop out in an orderly fashion, more or less separately, each in its own pond.

The great landmark of this process is the string or evaporation ponds-a salt farm, or more accurately, a solar-powered factory. The oldest of these salt refilleries is not at the lake but at the salt flats. There a salt farm has been extracting potash, for fertilizer, from the deep strata deposited by Lake Bonneville during the Ice Ages. Other projects are in operation along the shores of Great Salt Lake, on a still bigger scale.

So this scruffy tiling that people disliked has sud­denly become a pot of gold. Its mud-flat shores are becoming lined with huge salt farms. In the ground view they won't show up much, but for air travelers they will be the biggest man-made feature in the land­scape. Because the chemistry of the brine in the man-made solar evaporation ponds differs from bed to bed, each bed shows its own characteristic color in the sunlight: blue, brown, silver, purple, milky-like a collection of enormous stained-glass panes strewn across the surface of the desert.