The geologic map is good for understanding the underlying geologic formation but does not well address the surface soils upon which the overflow spillway is constructed. Below is a surface soils map with suitability for shallow excavations characterized. As can be seen, the suitability rating is 1.0 for most of the surface soils present at the spillway. A 1.0 rating indicates the surface soils are unsuitable for shallow excavations due to the presence of soft and hard bedrock. Hence, the overflow spillway was likely constructed on soft and hard bedrock.