Mystery objects 2

On a walk in March last year I came across a pile of hard, red brick-coloured spheres, about 1/4" to 1/2" diameter, on the ground. They seemed hollow as they were light. I took a couple of photos, and a few of the balls, and forgot about it. Then a month later. on our walk of the Monmouthshire and Brecon Canal I found some more. I don't know what they are. Can anyone help?


what are they?


a finger for scale


High Bridge 62, Mon and Brec, where I think it was that I found some more