The City of Northfield has a Park System Master Plan and on page 5 of the subsection 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)

Here is the 1999 Concept Drawing and Planting List for the Valley Pond Park.  Also included is a list of birds seen in the park (year?), drawn up by Don Starkweather.