Question

The following passage is adapted from an essay on mid-twentieth-century oceanography and cartography.

In 1952, geologist Marie Tharp was tasked with analyzing soundings collected by research vessels across the North Atlantic Ocean. Because women were prohibited from aboard research ships at Columbia University's Lamont Geological Observatory, Tharp relied entirely on continuous depth profiles recorded by echo sounders and transcribed onto paper rolls by her colleague Bruce Heezen. While plotting six east-to-west profiles across the ocean floor, Tharp identified a distinct V-shaped notch that descended nearly 10,000 feet below sea level at the center of a continuous undersea mountain chain. She hypothesized that this central depression was a rift valley—a continuous rift where the ocean floor was pulling apart. When she presented her finding to Heezen, he initially dismissed the rift valley hypothesis as 'girl talk,' attributing the V-shaped notch to measurement errors in the echo sounding instruments. It was not until 1953, when Heezen correlated the location of earthquake epicenters mapped by seismologist Maurice Ewing with Tharp's profile coordinates, that he recognized the earthquake epicenter points aligned precisely within the V-shaped valley. This alignment provided independent confirmation of Tharp's rift hypothesis, leading to the publication of their first physiographic map of the North Atlantic in 1957.

Based on the passage, what did Bruce Heezen initially believe caused the V-shaped notch identified on the ocean floor profiles?