Monument Hill (Palmer Divide)

Monument Hill is the high point on I-25 between Denver and Colorado Springs, with a summit elevation of 7,352 ft. It sits on the crest of the Palmer Divide, a caprock ridge that splits the Arkansas River basin to the south from the South Platte basin to the north. Drivers and locals also call it the Black Forest Divide Pass or Monument Hill Divide. The 18-mile stretch of I-25 between Castle Rock and Monument carries the nickname "The Gap."

This is not a single steep pitch. Heading north from the summit, I-25 sheds over 1,000 ft of elevation across those 18 miles, so it reads as a long moderate grade rather than a wall. Castle Rock sits at 6,202 ft, roughly 1,150 ft below the top. No official percent-grade figure for the steepest section was found, so go by the elevation change and plan for a sustained climb either way you take it.

Truckers care about Monument Hill because it is the highest sustained grade on the Denver to Colorado Springs corridor and it sits in an upslope snow zone that can dump far more snow than the cities at either end. It is a recurring site of multi-truck slideouts and winter closures. During the I-25 South Gap rebuild, CDOT added truck climbing lanes near Monument Hill and the Greenland exit, plus a southbound chain-up station in Larkspur so drivers can hook up before the climb.

• Summit elevation is 7,352 ft, the highest point on I-25 between Denver and Colorado Springs (Wikipedia, citing CDOT segment list; AARoads)
• I-25 drops over 1,000 ft from the summit across the 18-mile "Gap" between Monument and Castle Rock (AARoads)
• Castle Rock sits at 6,202 ft, about 1,150 ft below the summit (AARoads)
• Colorado's Must Carry Law requires commercial vehicles over 16,000 lbs to carry chains Sept. 1 to May 31 (CDOT, Oct. 2025)
• The state has 130 designated chain stations across more than 1,400 centerline miles (CDOT, Oct. 2025)
• In the March 13-15, 2024 storm, NWS Boulder logged 20.0 to 25.7 in near Castle Rock and 25 to 28 in at Sedalia (NWS Boulder)
• The I-25 South Gap rebuild ran Sept. 2018 to Nov. 2022 and cost $419 million (CDOT South Gap project page)

There is no scheduled seasonal closure here. I-25 is an interstate and stays open year-round in normal weather. The chain rule runs on the calendar though: Colorado's Must Carry Law requires commercial vehicles over 16,000 lbs combined weight to carry chains from Sept. 1 to May 31. When CDOT posts Code 18 (Level 2), all commercial vehicles must chain up, and single or tandem drive-axle combos chain four drive wheels. Code 18 can go up any time snow covers the full traveled pavement on an ascending grade. Closures come storm by storm, not on a schedule. When semis slide out or visibility drops, CDOT and CSP impose chain laws, trailer restrictions, or a temporary full closure, and those usually clear in a matter of hours, often overnight, rather than days.

Before the interstate, the main north-south route through here ran on the State Highway 105 corridor through Monument and Palmer Lake, then up through Larkspur toward Denver. Most of the corridor growth on this stretch has come since 1990. The Palmer Divide itself is named for Gen. William Jackson Palmer, the founder of Colorado Springs.

The current road is the product of the I-25 South Gap project, an 18-mile rebuild from south of Castle Rock to Monument. Work began in Sept. 2018 and finished in Nov. 2022 at a cost of $419 million, funded with money from Douglas and El Paso counties, the Pikes Peak RTA, and a federal INFRA grant. The project added an express lane that opened Dec. 10, 2021, truck climbing lanes near Monument Hill and Greenland, and the southbound Larkspur chain-up station. More recently, on Jan. 2, 2025, CDOT began installing fiber so the chain-law signs at the southern-Colorado chain stations can be flipped on remotely instead of plow crews stopping to do it by hand, with completion targeted for spring 2025.