Final answer:
The Sierra Nevada and Cascade mountains represent significant geographic barriers that inhibit movement from Washington, DC to California, due to their challenging terrain and weather conditions.
Step-by-step explanation:
The geographic barrier that would inhibit movement from Washington, DC to California is the combination of the Sierra Nevada and Cascade Mountain ranges.
These ranges are significant due to the rain shadow effect they create, which can lead to semi-arid conditions on their leeward sides. Traveling across these ranges would be challenging for several reasons, including their sheer size, the rough terrain, and potential for adverse weather conditions.