The City of Northfield has a Park System Master Plan and on page 5 of Appendix A, Grant Park to Hidden Valley (PDF), are the details for Hidden Valley Park with a photo (click to enlarge). It reads:
Hidden Valley Park
Classification: Conservation (Nature) Area — 6 acres
Hidden Valley Park
Character: This is a natural, open space park centered around a stormwater management pond. The only development is an asphalt trail, which connects to the local streets and larger sidewalk and trail system. Most of the space around the pond is naturalized, which is appropriate and could be further expanded to reduce maintenance costs and improve wildlife habitat.
Development Considerations: Expand naturalized plantings, with an emphasis on trees.
Potential Development Costs to Optimal Level: $5,000 – $10,000.
Other parks adjacent to Hidden Valley Park:
Heritage Park (south/southwest)
Roosevelt Park/Roosevelt Ridge Park (east/southeast)
Truman Park (northeast)