WOODSIDE, Calif. — After racing and biking back roads on the San Francisco Peninsula for almost half a century without serious incident, on July 3 I crashed while riding downhill at more than 30 miles an hour. I knocked myself out, broke my nose and was left with lots of road rash and stitches on my face, as well as a deep gash above my knee that went all the way to the patella.

But when the paramedics picked me up and sent me to the hospital, I found that the worst thing I had sustained was a hole in my memory, lasting about 20 minutes. I had been riding by myself, and I had no idea what led me to crash.

I might have hit a pothole and been thrown off my bike. Worse, I might have passed out before crashing, a possible sign of a serious underlying condition.

What I do remember is this: Riding back from the Pacific Ocean after a long climb on a hot day, I started downhill. Some time later, I recall coming to, staring up at redwoods.