1. The Appalachian Mountains: Formed over 480 million years ago during the Paleozoic Era, the Appalachians are a vast, ancient mountain range that stretches from Alabama to Maine. They are a testament to the Earth's tectonic activity and have witnessed countless geological changes throughout their history.

2. The Ozark Mountains: These mountains, located in Missouri and Arkansas, are around 500 million years old. They are also part of the Appalachian system, though they are a distinct plateau region known for its rolling hills, forests, and caves.

While these two are the most widely known, other ancient landforms also exist within the US. However, determining the exact "oldest" can be tricky as geologic history is complex and often requires specific dating techniques.