Geological Chronicle
Horrocks sits directly inside the degraded northeast rim of Hipparchus, an ancient walled plain whose ruined structure forms its immediate frame. The impact structure spans 29.65 kilometers across and cuts approximately 3 kilometers down into the surrounding terrain. It took shape during the Eratosthenian period, a geological span between 3.2 and 1.1 billion years ago, long after the host basin had weathered.
The crest of the crater follows an irregular, polygonal outline marked by a distinct outward protrusion along its eastern wall. Downward motion has heavily reshaped the interior, where the inner slopes are slumped and accumulate in a prominent heap of talus along the northwest. Across the uneven floor, the center rises into an isolated mountain flanked by smaller hills.