Iron Concretions like these go through complex changes over time to form iron rich stone. They might start as clay, or another sedimentary material, and harden in layers over time, turning the iron-rich mineral into a heavy lump of layered iron stone. 
18 - 20th century Scottish miners relied heavily on smashing rocks apart to get iron rather than sourcing rarer deposits. The Scots and northern English invented multiple mining techniques to make this process easier and faster.
A great place to learn about the mining industry in Scotland is down in Wilsontown. It's a nice walk around a rewilded pit. Obviously the Mining Museums are good too if you prefer indoor entertainment.
These iron Concretions came from the waterways of South Lanarkshire where mining was the bread and butter for thousands of families. We'd say millions, but Scotland is small. Maybe one million families...