On these hot, dry June days when the horizon shimmers, set to dancing by the waves of heat that rise from the ground, I think of beaches. Not ocean beaches - playas -desert beaches. Playas are the dry, incredibly level beds of ancient lakes. Found in desert country throughout the southern Southwest and northern Mexico, and the Great Basin country of western Utah and Nevada, normally-dry playas occasionally fill with a skim of water - sometimes no more than inches deep over many square miles - after a heavy summer rain or spring snowmelt. Such lakes never last more than days or weeks, soon evaporating to leave behind huge expanses of mudflats drying in the sun. Playas were named by Spanish explorers for their resemblance to beaches - very flat beaches.

Playas are characteristic desert landforms, maintained by climates drier than they are wet, where the combination of relentless sun, wind, and thirsty air can evaporate many more times moisture from the land than falls on it in a year. Most of the Southwest's playas formed during our last extended rainy season, many thousands of years ago. During the Pleistocene, the glacial era in the Northern Hemisphere, the climate in the Southwest was cooler and much wetter. With more precipitation, streams and rivers abounded. Yearround streams rushed down now-dry drainages in the mountains, eroding boulders, cobbles, gravel, sand, and silt as they ran, dropping the larger

, heavier sediments where the water slowed down at the edge of the mountains. The basins between mountain ranges filled up with finer sands and silts, atop which floated shallow lakes.

As the climate warmed and dried some ten to twelve thousand years ago, the streams dried up and the basin-filling lakes evaporated, leaving their flat-floored beds to dry and harden to a cementlike consistency. The beds of these long-vanished lakes, including the Lordsburg playa and Lake Lucero, the playa that supplies gypsum sand for White Sands, are today's desert playas.

When the lakes return briefly, the profusion of salty water teems with tiny aquatic lives: algae, freshwater shrimp, brine flies. These suddenly appearing residents survive the intervening months or years of drought as eggs or encysted larvae in the cracks of the dry playa, waiting for water's next blessing.

When the water evaporates, it leaves acres of shiny, gooey mud flats, level as the surface of a pool table, encrusted with a new coat of calcium, sodium, gypsum, and other salts from the recently-departed water. The mud surface dries into curls, pie-wedge-shapes, cylinders, or shardlike plates. Polished by the ever-constant wind, it sometimes shines like glass. The flat expanses are exhilarating, Ann Zwinger says in The Mysterious Lands, "I stand, like the pivot point of a compass, in the center of the universe-a place to dance, to hoot and holler, the rearrange mountains, to count the rollicking stars at night."