Language models rely on increasingly large web-scraped datasets for pretraining. The size of these datasets prevents manual curation, and existing automated quality filters are heuristic and limited. Characterizing these datasets is an open problem. We present preliminary work on documenting and visualizing pretraining datasets by mapping their similarity to downstream benchmark datasets, which are often hand-curated and more focused in style and content. We show this method finely characterizes popular pretraining datasets, supplementing existing characterizations that can be used for quality filtering.