Quaternary
alluvium covers the main western part of Pialligo. Calcareous
shales from the Canberra Formation are deep underneath. The Woolshed Creek runs into the north end of Pialligo. This is a significant place, because in the bed of the creek the Rev W B Clarke first recognised Silurian fossils. These fossils were
brachiopods, mostly
Atrypa duntroonensis. He discovered them around 1844 century and it was the first time that Silurian rocks were identified in Australia, and at the time were the oldest known rocks in Australia. This
mudstone is from the Canberra Formation. ==Living history==