The Earth Keeps Making Mountains in Colorado and Scientists Don't Know Why

TL;DR

Colorado has hosted two separate mountain ranges because tectonic forces 300 million years apart repeatedly deformed the same inland region for poorly understood reasons.

Key Points

• 1.A 1.4-billion-year gap in Colorado's rock record reveals a massive erasure of geologic history. At Red Rocks Park, rocks dated 1.7 billion years old sit directly next to 300-million-year-old sandstone; one hypothesis blames Snowball Earth glaciations (717–635 Mya) scraping away up to five kilometers of rock.
• 2.The Fountain Formation's red sandstone is direct evidence of the vanished Ancestral Rocky Mountains. Its arkose sandstone contains feldspar, granite, and gneiss fragments — materials sourced from fast-flowing rivers draining steep mountain terrain that existed ~300 million years ago.
• 3.The Ancestral Rockies formed ~323 million years ago as Pangea assembled, far from any plate boundary. They rose 1,500 km inland, likely because the Africa–South America collision sent ripple-like deformation across North America, or reactivated an ancient fault system called the Wichita Megashear.
• 4.Scientists genuinely don't know why mountains keep forming in Colorado's interior. Both the Ancestral and Modern Rockies formed far from tectonic plate boundaries — a location that defies standard mountain-building models — and no consensus explanation exists for either range.
• 5.The Ancestral Rockies eroded completely by ~175 million years ago, but left a lasting legacy. Their sediment filled basins across North America, creating the geological conditions that trapped the oil and gas deposits humans later exploited.