This has been one of the strangest and most emotional summers of my adult life, and I'm not even really sure why. Well, I know part of it.

Now it's ending, the summer, and North Dakota is grinding inexorably forward towards first frost. I'm filled with such a palpable sense of loss that it sometimes makes me put my head down the way an abscessed tooth does.

It's not that I want the summer to reboot, but sometime in the last couple of years the realization has come to me that there is a limit to the number of harvests I will witness, a finite number of Thanksgivings, even a finite - and diminishing - number of new moons to come.

Almost every time I see a new moon I pick up the phone and call my daughter. When she answers, I tell her to "drop whatever you are doing and go out to look at the new moon." She is usually pretty gentle when these moments come, but I can hear a little hint of "there he goes again" in her response.

You see, at 15 she believes the world is infinite. So far she regards time as a mere organizational device, not a muted death sentence, a slow-mo passport to "the undiscovered country from whose bourn no traveler returns," as the gloomy Hamlet puts it. So far she does not believe in death. It has not percolated into her sheer animal exuberance at life, her urge to fast-forward everything (driving, curfews, college, her first car). She still announces her age by way of whatever fraction of a year has accrued since her last birthday.

Ah, my lovely lyrical child. I have no Calvinist urge to set you straight. Linger on in all the whimsies of the world.

My parents' best friend who, once he had turned 60, used to say, "Today I bought my last pair of shoes." He was, of course, joking in a mock-morbid way, and he lived for a couple of decades after his first use of the gag. But at some point, of course, he did buy his last car, his last shirt, even his last bottle of wine.

Why all of this shadow should be cascading in on me in the midst of a glorious autumn is another question. Maybe it's that last year reminded us what can follow autumn in this part of the planet. Maybe it's my left shoulder, which, thanks to something called a bone spur, wakes me up at night. Mostly, it's the cluster of 40th anniversaries we've been observing.

Historians sometimes call 1968 "the year that rocked the world." That certainly makes sense, given the Tet Offensive, the assassinations of Martin Luther King and Robert Kennedy, the anarchic Democratic Convention in Chicago, President Johnson's decision not to seek re-election … But 1969 was a watershed year, too, one of the momentous years that somehow linger in the historical imagination.

We have been observing searing 40th anniversaries this summer: the Apollo 11 moon landing; Chappaquiddick (made the more poignant by the death, on Aug. 25, of Edward Kennedy); the Manson murders; the Mother of All Rock Concerts, Woodstock; and, here in North Dakota, the Zip to Zap (May 9-11, 1969).

These events are important cultural markers of the tumultuous years. We live now at a time when the explosion of major media makes every historical anniversary an audiovisual feast. But these 40-year anniversaries have a particular bite for Americans over 50, of which, alas, I am one. There are more than 50 million of us, each of whom was at least 10 years old when Neil Armstrong jumped down onto the surface of another world or when Jimi Hendrix turned the Star Spangled Banner into a screaming feedback anthem of American disillusionment on Aug. 18, 1969, at Woodstock.

But wait. This little demographic analysis reminds me that if for 50 million Americans 1969 is one of the pivotal years of our lives, then for 260 million others it is just a year long ago in a galaxy far away in which lots of big things happened. Which means that most Americans, including perhaps most of you reading these words, justly wonder what all the fuss is about and why I am being so morbid.

I'm not sure I can explain it, but here's a piece of it. My daughter is, in 2009, almost precisely the same age I was when all this happened. Those historical-cultural events of 1969 were among the first that really got into the heartland of my consciousness. They marked my life so deeply that they have never just slipped off the edge of the stage and into oblivion. I have never been able to drag them to the wastebasket icon in the lower right corner of my soul and "empty the trash."

The Vietnam War protests, the Battle of Hamburger Hill, Joe Cocker's "With A Little Help From My Friends" performance at Woodstock, the technological triumph of Tranquility Base, the Beatles' rooftop "Let It Be" concert (not to mention the breakup of the Beatles) are central episodes of the core narrative of my adolescence. They loom large for me (and millions of others) because they prick that raw spot in my upbringing when the complexities of the world could no longer be ignored, when the erotics crowded in on simpler appetites, when I (we) first began to puzzle the gap between our dream of life and what life actually is, when we first asked why America is not always quite what it pretends to be. When we first became so restless.

If these things marked me so fully at 141/2, then what is marking my daughter's consciousness at 151/6? I'm not sure I am ready to face that. And what will her nostalgia-anxiety-morbidity reveries look like in the year 2049? This morning I was writing a letter to her seemingly unrelated to all of this. She has recently won a significant school award and I was trying to explain the "Butterfly Effect" - the idea that something seemingly unmomentous can create a "disturbance" that has monumental impact far down the road in a faraway place.

I employed a river metaphor: I love rivers, especially the Little Missouri River after which she is named. I was straining to explain to her that she is still a mountainside freshet near the source finding its way down onto the alluvial plane and that her path and the kind of cargo she will carry is being determined in these second-decade years of her life.

After struggling with that idea for a while, writing and revising, changing a word here and a phrase there, I stepped back and read the paragraph start to finish. It had, it seemed to me, a kind of tedious beauty to it, but I knew for her the emphasis would be on tedious. Or perhaps turgid.

It's the incommunicability of loss (or ecstasy) that eats at one's life.

(Clay Jenkinson is the Theodore Roosevelt Center scholar at Dickinson State University, as well as Distinguished Scholar of the Humanities at Bismarck State College. Clay can be reached at Jeffysage@;aol.com or through his Web site, Jeffersonhour.org.)