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International Ocean Discovery Program (IODP) Expedition 374 recovered high-quality cores at five sites on the Ross Sea continental shelf, slope, and rise to improve the understanding of the sensitivity of the Antarctic ice sheets (and particularly the West Antarctic Ice Sheet) to past climatic and oceanic conditions, especially during a warmer-than-present climate. This report summarizes the petrology of gravel-sized clasts from Site U1521, which is located in the Pennell Basin. The recovered core spans from the early Miocene to the Pleistocene, and it is constituted by cycles of glaciomarine sediments that indicate different paleoenvironmental conditions. Granule- to cobble-sized clasts present in the sedimentary sequence have been counted and grouped into seven different lithologies based on macroscopic and microscopic recognition. The most common lithologic group is represented by low-grade metasedimentary rocks such as metasandstone, metasiltstone, and metagraywacke. Granitoid rocks (mainly monzogranite to granodiorite) are the second most represented group. Dolerites and volcanic rocks are less frequent and are abundant only in some lithostratigraphic units. Chemical analysis of biotite from seven selected metamorphic and intrusive pebbles are also provided.