Models point to a continent-long cliff as the cause of the Grand Canyon's missing billion years

The round figure undersells the gap. From 1.8 billion to 520 million years ago is about 1.28 billion years ^[18], all of it missing from a canyon whose layers otherwise run back about 2 billion years ^[1].

The case rests on simulation. Gernon's group modeled how ancient Earth changed over hundreds of millions of years and watched an escarpment rise at roughly the time the region's rocks disappeared ^[5]. In the reconstruction, the cliff began forming about 800 million years ago as the supercontinent Rodinia came apart ^[4]. A match in timing inside a model shows the cliff could have done the job, and Gernon pitches his claim at that level. "No study has previously suggested the existence of a continental-scale escarpment in the Grand Canyon region. Our study shows it's a plausible, straightforward explanation for the immense erosion that took place in the Grand Canyon," he said ^[7].

The proposed erosion works two ways. A cliff 1 to 2 kilometers high ^[6] leaves steep slopes where rain and ice can loosen exposed rock ^[8]. Its height would also have favored glaciers once Snowball Earth set in ^[10]. "The exposed rocky peaks of the escarpment would have acted as glacier factories," Gernon said ^[11]. He made a similar argument in a separate paper in Science, where his team linked a tectonic escarpment to the start of the East Antarctic Ice Sheet some 34 million years ago ^[12]. The team also cites present-day escarpments in Antarctica and South Africa where similar erosion is under way ^[16].

The Science News account does not describe which competing explanations the model was run against, or how its output was checked against rock beyond the canyon. The open question Gernon names is timing. Nobody can yet say whether the rock was stripped before, during or after the Snowball Earth glaciations ^[13]. "That's the real hole that needs filling: pinning down the erosion history in space and time, continent by continent, precisely enough," he said ^[14].

I think the work supports Gernon's own word, plausible ^[7]. A dated erosion record of the kind he describes is what would turn it into an answer ^[14].

Alan Collins, a geologist at Adelaide University who was not involved, said tying the vanished rock to independently verified events such as the breakup of Rodinia bolsters the case ^[15]. "I find this very convincing," Collins said ^[17].