1. Shale: Shale is a sedimentary rock formed from compacted clay and mud. It has a layered structure and breaks easily along these layers, making it quite brittle.
2. Sandstone: Sandstone is also a sedimentary rock, composed of cemented sand grains. While some types of sandstone can be quite strong, many are relatively soft and prone to crumbling, especially when exposed to weathering.
There are many other rocks that could fit this description depending on the specific type and its formation conditions, but shale and sandstone are two common examples.
