Which observations led Louis Agassiz to conclude that glaciers had once covered much of Switzerland?

Louis Agassiz concluded that glaciers had once covered much of Switzerland and beyond by identifying specific geological features that contradicted the prevailing "diluvial" theory, which attributed such phenomena to ancient floods.1

His conclusions were primarily based on the observation of three key pieces of evidence:

• Erratics: These are large rock fragments, such as granite boulders found resting on limestone in the Jura Mountains, that differ from the local bedrock. Agassiz demonstrated that these were transported by glacial ice rather than water torrents.12
• Till: He identified sheets of unstratified sediment, known as till or drift, spread across the landscape as evidence of glacial deposition.12
• Striated Pavements: He observed rock surfaces marked by striations, which provided further proof of the grinding and movement of massive ice sheets.1

Through his intensive field studies, published in his 1840 work Études sur les glaciers ("Studies of Glaciers"), Agassiz successfully argued that Alpine glaciers had been significantly more extensive in the past, effectively establishing the scientific foundation for the ice age theory.12