Observatory Hill

State Natural Area (No. 223)

Location: Marquette County. T14N-R10E, Section 8. 175 acres.

Access: From the intersection of Highways 22 and F on the south side of Montello, go south on F 4.8 miles, then east on 14th Road 0.5 mile, then south on 13th Road 1.4 miles, then east on Gillette Avenue 0.6 mile to a parking area north of the road. A footpath to the summit begins at the northwest corner of the old field.

Description: The highest point in Marquette County is Observatory Hill, an isolated outcropping of porphyritic rhyolite rising 300 feet above the surrounding landscape. The fine-grained rhyolite, an igneous rock embedded with feldspar crystals, is highly resistant to erosion and has been dated to 1.76 billion years ago. On the exposed rocks at the summit are polished rock surfaces with grooves, striations, and chatter marks created by the passage of the glacial Cary icesheet 12,000 years ago. Recently, scientists have discovered the existence of petroglyphs on the hill’s rock outcroppings, which may be part of a larger prehistoric petroform found in Marquette County. The slopes are covered with a southern dry forest community dominated by red and white oak, basswood and shagbark hickory and much of the area is now being restored to oak savanna – an imperiled vegetation community in Wisconsin. Near the top of the hill, bedrock is exposed or close to the surface creating acidic conditions where a specialized glade community has developed. Red cedar dominates the glade and the thin soils support a sparse ground cover of mosses, ferns, and lichens. The state-threatened slender bush clover (Lespedeza virginica) is also found on the site. The hill has long been known by naturalists and was a favorite childhood haunt of John Muir who lived nearby. Observatory Hill is owned by the DNR and was designated a State Natural Area in 1989.