For the past twenty years I have spent a June week on Nantucket with my mother and my kids. (Pastels and scrimshaw are not my husband’s cup of tea.) Those annual trips became, like height marks on the kitchen door, a measure of my children’s growing up. Our bike-riding history reflects their progress from one plateau of independence to the next. In the early years I left the baby at the cottage with my mother, and pedaled gingerly along with my toddler strapped into a child seat behind me. We graduated to small bikes and short rides. I still smile at the memory of how we must have looked for a year or two, the proud mother riding ahead with two furiously pedaling goslings in a staggering row behind. My favorite stage was also the shortest lived: when we were confident cyclists on ten-mile tours, one or another pulling ahead and meeting up at predetermined rest stops to compare notes on the ride so far.

Soon, though, I was left in the dust. My children would disappear over the horizon, sometimes doubling back to make sure I was still okay or heading off into town on their own. While I kept pedaling, they moved on to scooters and dune-riding in a rented jeep – it all went by so fast. I now bike alone.

This summer I realized that while I was concentrating on how my kids were growing up, I had missed the evidence that my mother was growing down. In my recollections she was a constant ingredient in the Nantucket experience - an enthusiastic browser in the tourist shops; a devotee of Aunt Leah’s fudge; and events manager, spending hours combing the local paper for auctions, star-gazing, and amateur theater productions. She was always fascinated by new information about the island’s whaling history. Then she wasn’t.

Looking back, I now see that somewhere along the way she stopped driving the car to meet us at our biking destination. Somewhere else along the way, I began to feel discomfort at leaving her at the beach with the kids while I went grocery shopping. Her walk slowed, and the cobblestones threw her off balance. Then last year I left her in a shop picking out souvenirs for her friends, and when I came back she was standing where I had left her, staring into the same basket of bracelets.

This year she didn’t know where she was. “Whose house is this?” she asked over and over. “Where are the kids?” As the days went by she occasionally recognized the place, and when she didn’t, it must have been somewhere in her mind, because she kept suggesting that we “go to Nantucket for a visit soon.” She sat on the deck happily sniffing the damp air and watching the trees blow in the wind. She no longer expressed any interest in going into town, and when we went out to dinner, she seemed agitated by the unfamiliar sounds and crowds. But she enjoyed the meals we cooked back at the house and lingered over the sweet corn until the last kernel was nibbled. She didn’t know where she was, but she was happy to be there.

As the ferry pulled out of the harbor on our last morning, we joined the other passengers on deck tossing pennies into the gray water lapping at the Brant Point lighthouse – a wish and a promise to return to the island. Only this time my mother wasn’t at the railing; she was inside looking placidly out the window. We each threw a penny for her. But I doubt she will be back.

The fog closed in as we headed out to the open sea, and I thought about growing up and growing down. My mother is drifting toward a deeper and deeper fog. My kids, on the other hand, are heading away from the shore of their childhoods toward a new world; it is as though they are still on their bikes, way ahead of me in the salty mist – on their own expeditions of discovery.