1. Metamorphism: Precambrian rocks are often heavily metamorphosed. This means they've been subjected to intense heat and pressure deep within the Earth's crust, changing their original mineral composition and structure. This makes them very hard and resistant to erosion.

2. Limited Fossil Record: The Precambrian eon spans a vast amount of time, but life during this period was mostly single-celled and soft-bodied. These organisms rarely left behind fossilized remains. As a result, Precambrian rocks contain far fewer fossils compared to younger rocks, making it more difficult to study life from that era.