How was the sedimentary rock that underlies the lowlands formed?

Answer 1

When the mountains got eroded, sediments broke off, and rolled down the mountain into the lowlands, that were oceans at the time, and the pressure from all the rocks fused them together, making it a sheet of sedimentary rock

Answer 2

The sedimentary rock underlying lowlands is typically formed through the accumulation and lithification of sediment particles, such as sand, silt, and clay, over long periods of time. These sediments are often deposited in low-lying areas like river valleys, lakes, or coastal plains, and as they become compacted and cemented together, they form sedimentary rocks. Over time, these processes create layers of sedimentary rock that can be uplifted to form lowlands.