Rock Layers Beneath the Soccer Field

After the Storm at Harborview Soccer Field

A strong rainstorm passed over Harborview Soccer Field. Water flowed across the grass toward a drainage ditch beside the south goal. The flowing water carried some loose soil away and cut a shallow trench. Workers placed a safe barrier around the trench while a geologist studied the exposed rock. The storm changed the surface of the field, but it did not form the older rock layers in a few hours.

The geologist recorded two undisturbed rock cuts. In an undisturbed stack, a lower layer formed before a layer above it. The South Cut had these layers from top to bottom: S4, brown silt with a few modern grass roots; S3, dark mudstone with fossil leaf impressions and casts of tree roots; S2, tan sandstone with fossil freshwater clams and fish scales; S1, dark shale with fossil shells from animals that lived in shallow salt water.

The North Cut showed a similar pattern, although its layers had different names. From top to bottom, N3 was mudstone with fossil leaf impressions and tree-root casts; N2 was sandstone with freshwater clam fossils and fish scales; N1 was shale with shallow-saltwater shell fossils. The cuts were far enough apart that the geologist did not assume every layer had the same thickness.

The geologist used the fossils as clues about old environments. Saltwater shells suggest a shallow sea, freshwater clams and fish suggest a lake or other freshwater setting, and tree roots and leaf impressions suggest land where trees grew. The soccer field today is grassy, but today's surface does not show what every older layer was like.