The Gable Creek formation grades from a high energy fluvial system to low, which is shown in the rock record trend from large volcanoclastic material (volcanic in origin river cobbles, and other larger rocks) to softer marine alluvial fans. During the Cretaceous, Eastern Oregon was either part of a shoreline or under the ocean. The Goose Rock Conglomerate, with its imbricated (stacked in the direction of flow) and diverse well-rounded cobbles tell us that a high energy river emptied into an ocean basin near the National Monument approximately 88 million years ago.
Gable Creek Formation
The Goose Rock Conglomerate (GRC) found within the boundary of the Sheep Rock unit is a portion of the Gable Creek formation (GCF). The GCF encompasses a wide swath of late cretaceous river deposits trending East-Southeast. The conglomerate outcrop in the monument, with its well-rounded river rocks stuck in a sand stone matrix, would have been near the mouth of a large river and looked similar to a modern delta, like the Nile delta in Egypt. Though there haven’t been any dinosaur fossils found in the park from this formation, further down the geologic section there was a plesiosaur excavated from a similar unit of the GCF. The overall Gable Creek Formation portrays trends of high energy fluvial systems (river deposits) to marine alluvial fans (mudstones and siltstones).