(MONEY Magazine) – Several years ago I embarked on a lengthy cross-country drive. I tend to be rather statistics-oriented--okay, geeky--so when I got home and friends began asking about the trip, I was ready with a litany of numbers, which I became very adept at reciting: "Well, I drove about 14,000 miles, passed through 25 states, visited five national parks and three national monuments, ate at 14 different barbecue joints..."

"Wait a minute, back up for a second," said one acquaintance, interrupting the seamless flow of my delivery (the nerve!). "What's the difference between a national park and a national monument anyway?"

I blinked, thought for a moment, and realized I hadn't the slightest idea. Prior to the trip, I think I instinctively assumed that national monuments were man-made structures, like the Washington Monument. But none of the national monuments I'd just visited fit that description. In fact, they seemed very much like national parks, offering many of the same amenities (hiking, camping, gorgeous landscape, irresistibly cute prairie dogs). So what exactly was the difference, classification-wise, between Badlands National Park, where I'd stopped while passing through South Dakota, and Devils Tower National Monument, which I'd visited the following day in Wyoming?

My interest piqued, I soon learned that both national parks and national monuments are run by the National Park Service, whose domain also includes national memorials, national historical parks and over a dozen other facility designations. There are 73 national monuments, compared with only 57 national parks. Why the disparity? Because most NPS units can be established only by an act of Congress. But the Antiquities Act of 1906 allows national monuments to be created simply by presidential proclamation, in order to preserve "historic landmarks, historic and prehistoric structures, and other objects of historic or scientific interest upon lands owned or controlled by the United States." Bill Clinton, hoping to secure his environmental legacy--and with considerable controversy--invoked this power nine times during his final four days in office, and 14 times earlier in his presidency, making him history's most monumental President, so to speak. (See the box on page 136 for a look at other national monument imbroglios.)

As I had initially surmised, some national monuments are structures--the Statue of Liberty, for example, and a handful of old forts. But most national monuments are natural sites, many of them focusing on highly specialized landscapes. The Antiquities Act specifies that monuments "shall be confined to the smallest area" necessary for protecting and managing the site, so they tend to be smaller than national parks, making them ideal if you're short on time or if someone in your family is short on patience. And since national monuments are less celebrated than national parks, they appeal to my love of the overlooked, the inconspicuous; on a more practical level this also means they usually aren't too crowded.

Three months after the Antiquities Act was passed, Theodore Roosevelt became the first to invoke it, creating Devils Tower National Monument (Devils Tower, Wyo.; 307-467-5283; www.nps.gov/deto). Nearly a century later, Devils Tower is now probably best known for its starring role in the movie Close Encounters of the Third Kind, and it's easy to see why it worked so well as a dramatic symbol: The remarkable 1,267-foot tower, with its ridged edges, conical slope and plateaued summit, looks almost too premeditated, too designed, to be a natural formation. You can judge for yourself on a series of beautiful hiking trails, which provide breath- taking views of the tower. Scores of bird and animal species make for great wildlife viewing, and the truly hardy can bond with the site by climbing the face of the tower itself.

Here are some of my other favorite monuments. Like Devils Tower, they each spotlight a unique type of landscape, making them perfect places to appreciate America's topographical diversity.

Organ Pipe Cactus National Monument (Ajo, Ariz.; 520-387-6849; www.nps.gov/orpi): Established in 1937 by Franklin D. Roosevelt, this amazing site lies against the Mexican border in southwestern Arizona, about a two-hour drive from either Phoenix or Tucson. This is the Sonoran desert, a starkly gorgeous region teeming with flora, including the monument's eponymous cactus, which is found nowhere else in America and can reach 25 feet high (and that's nothing--another cactus variety here, the saguaro, reaches 50 feet). Two dirt-road driving loops--one takes about two hours, the other about twice that long--provide spectacular views of the cacti and the majestic Ajo Mountains in the distance. It's a surreal place. After driving an hour or so, the cactus columns, many of them perfectly vertical, begin to seem like characters, and you feel as if you've stumbled upon a convention of very tall people with impossibly good posture. Visitors with less active imaginations will nonetheless appreciate the view at night, when the desert sky and remote location make for fantastic stargazing. Summer temperatures routinely rise into the triple digits (it's a desert, after all), so bring some water and make sure your car's air conditioning is working well.

Congaree Swamp National Monument (Gadsden, S.C., about a half hour southeast of Columbia; 803-776-4396; www.nps.gov/cosw): Most national monuments are in the West, in part because that's where the greatest federal land holdings are (the federal government actually owns 83% of Nevada, 63% of Idaho and 53% of Oregon). But this beautiful eastern site is an exception. And despite its name, it's not a swamp--it's a flood plain, home to America's largest remaining tract of old-growth bottomland hardwood forest. High timber prices in the 1960s made Congaree an attractive target for loggers, but the persistent flooding made commercial operations untenable, and the site was eventually protected by monument status in 1976. Ninety different tree varieties--half as many as in all of Europe--form a massive canopy, often creating a magical twilight effect, even at midday. Because the area floods about 10 times a year, the primary walking trails are on a series of raised boardwalks, allowing easy navigation through the beautifully verdant grounds. With hundreds of bird and animal species (and, at times during the summer, plenty of mosquitoes--bring repellent), this is one of the most biodiverse spots in America, and assorted lakes and creeks provide opportunities for canoeing and kayaking. A lush little pocket paradise.

Scotts Bluff National Monument (Gering, Neb., 3 1/2 hours northeast of Denver; 308-436-4340; www.nps.gov/scbl): Situated in the sparsely populated and surprisingly interesting region of western Nebraska (see "Right out in the Open," November 1998), Scotts Bluff, which was given monument status in 1919 by Woodrow Wilson, is one of several rocky outcroppings that served as natural markers on the Oregon Trail. The Trail itself passed right by the Bluff, and although the wagon wheel ruts are now gone (the best remaining ruts are about an hour's drive to the west, in Guernsey, Wyo.), the Bluff and its surrounding buttes are wonderful attractions in their own right. An excellent hiking trail beginning at the visitor's center takes you up a series of winding ridges to the Bluff's 425-foot-high summit, where you can survey the magnificent surroundings, take some can't-miss photos and imagine how the lay of the land looked to the original Oregon Trail pioneers, who hauled all their worldly belongings across this terrain at the rate of 15 miles a day.

Craters of the Moon National Monument (just west of Arco, Idaho and about three hours east of Boise; 208-527-3257; www.nps.gov/crmo): Established in 1924 by Calvin Coolidge and then enlarged by Bill Clinton in 2000, Craters of the Moon is a massive lava field, with more than two dozen volcanic cones and 60 distinct lava flows, ranging from 2,100 to 15,000 years old. A geologist's dream and a guaranteed head spinner for anyone else, the site looks like no place else you've ever been--barren, spooky, full of oddly beautiful rock formations. The seven-mile scenic driving loop is nice, but Craters needs to be experienced on foot, so get out of the car and explore the assorted hiking trails. Walking through the maze of basaltic rock, it's easy to imagine that you've traveled not across the country, not even across the world, but across space itself, for surely this bizarre terrain could not be Earth--not as we know it.

But of course the Earth's landscape is much more varied than most of us realize. Thanks to these and other national monuments, we have the chance to experience some of that amazing variety.

Paul Lukas still doesn't understand the difference between national battlefields, national battlefield parks and national battlefield sites, but he's working on it.