How quickly do objects typically erode on the seashore?

I’ve just spent a weekend in Southbourne, near Bournemouth (UK) - yesterday afternoon we went down to the beach and let the kids play, it is a beach of moslty yellow sand, but with a fair bit of flinty pebbles and shingle too.

I picked up a large triangular(almost perfectly equilateral) piece of green glass that had been worn by the waves/sand/stones, but the edges were still a little too crisp and unworn to make it worth keeping; I tossed it back into the shallows.

This afternoon, we returned to the same beach and in a spot not five yards from where I threw the piece, I found a large triangular(almost perfectly equilateral) piece of green glass that had been worn smooth, the edges were quite nicely rounded off, so I kept it.

It really did look like the same piece of glass - the shape and size was the same (but for the extra wear), although there is no way to be certain of course - it was just ordinary green bottle glass (of which there was not a huge amount in evidence)

So, my question; is it plausible that the piece I picked up and returned yeterday could have been perceptibly worn in the space of 24 hours? The weather conditions could be described as being flat calm, with very few waves big enough to break at the crests until they were met by a returning wave off the shore(although this might have roughened a little overnight).

OK, another way to ask it…Does anyone have a gem tumbler? - how much wear is evident on an object after 24 hours tumbling?

(while fully realising that the abrasives used are harder/sharper than beach sand and the tumbling action is continuous)

I do have a rock tumbler. And as luck would have it (for you anyway) I just emptied the latest batch on Saturday night. It takes four weeks of graduated steps to get a rock from rough to shiny - smooth. The softer silicon type rocks (like your glass piece) do wear faster and end up smaller than the rest of the stones.
I stop the process after one week of tumbling to clean the stones and add the next mixture of abrasive and the stones are already worn quite nicely. Kind of like the ones you`ll see on the beach.

I have never stopped the process after one day, so I cant tell you if its possible to get the result you think you saw. I can tell you that it takes about one week to get the rough edges off of a stone, and that is with an abrasive additive and with many other rocks tumbling together (about 30 small stones). I don`t think that with the ocean environment being mostly sand and the rocks not confined to one area (they get moved about more freely in the ocean) that you would expect to get “smoothing” in just one day.

Also, if the ocean were anything like my rock tumbler, most stones would be reduced to practically nothing in about two months. I say that because the stones coming out of the tumbler are about half the size they were when they went in (after one month).

So, I would say the answer is no.

Mangetout the answer is no. I have lived on the ocean my entire life, except 4 years for grad school in Arizona. The odds that the piece you picked up the first day , and the piece you picked up the second day are the same are so remote, it is almost inconceivable.


Glass usually gets tumbled under the soft sand quite quickly after you chuck it back in. Burried essentially. And during high tide the glass can shift up high enough, and close enough to the high-water mark - where the tide is the highest - to get left behind during low tide. This is when you find it.

It takes much longer than one day for glass edges to erode enough to make it sea-glass.

We have a tradition at my house, that every fall, when the tides get a little stronger, and the sand on the beach shifts about 100 yards off shore, We take a blue glass bottle. Smash it up in a bog, and chuck it into the water right off our beach. This way in the summer, people can find the coveted Blue sea glass on our beach…
Point to Ponder: There are more stars in the sky, than there are grains of sand on every beach on earth.

Phlosphr, you’re probably right; it is a big universe and coincidences just happen.

I should also have mentioned that the spot where I found/returned it was immediately adjacent to a groyne, had this not been the case, I would not for a moment have entertained the idea that it was the same piece. (without the groynes, the beach shifts sideways considerably with the tide).

I do hope you kept the piece of glass. Seaglass has a certain innate beauty, if in fact you did find the same piece twice, you are certainly blessed with cosmic synchronicity rock on Mangetout

I read somewhere that the average age of a seashell on a beach was thousands of years–but I can’t find a reference. Anybody?

Mentock - there’s no way unless it is a fossilized shell. Shell’s break up and go right back to becoming sand. Even clam and abalone shells, which are very hard.

Sorry, missed that. I’m not sure what you mean by fossilized shells–coquina for instance is original shells, compressed and fused. It takes a while for shells to become sand, the actual period is what is in question–but I’m not sure what you mean by “go right back”. I just googled, and found this article which found an average age of over 500 years in some California mollusk shells, and this article found 8000 year age in North Carolina oysters shells.