Data includes any facts, samples or statistics that are collected or recorded for later analysis.
His data included primary samples - that is, actual rocks that displayed sedimentary and other geologic traits. Keeping accurate records, descriptions and drawings of these samples also counts as data. In fact, his study was described as "so full of fossils and chemical apparatus that there is hardly room to sit down".
He also undertook field data collection. When rocks were too large to sample (bring back to the lab) or were part of a larger system or structure, field visits were required. Again, recording these visits with scientific observations count as data collection.
In particular were cliffs at Siccar Point - where vertical layers of shale met horizontal layers of bedrock. These cliffs formed some of his data collection that supported his hypothesis of geologic uplifiting.
This is what set his data about the history of earth apart from others and that made his ideas more credible than those of earlier theorists.