I recently visited Juneau, Alaska. It is a small city of 35,000. The city limits are quite large, however -- 3,225 square miles -- but much of this area is separated from downtown Juneau on Douglas Island. According to Wikipedia, Downtown Juneau sits at sea level, with tides averaging 16 feet (5 m), below steep mountains about 3,500 feet (1,100 m) to 4,000 feet (1,200 m) high. Atop these mountains is the Juneau Icefield, a large ice mass from which about 30 glaciers flow. Because the city is surrounded by ice or water, there are no roads in or out of town, and the only way to come or go is by air or sea. Given this, could the glacier ice and the ocean be considered like mechitzas (walls) and enclose the city like an eruv?
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It would depend on the exact dimentions of the barrriers. For Natural Topagraphy be considered a Mechitza, It would have to rise 10 tefachim in a span of 4 Amos. (mishna berura siman 363) (about 12% grade, if i remember my calculations correctly). To make a Mechitza for a whole town you would have to establish that there is a ring of natural mechitzos, following all the usual rules of connecting mechitzos and gaps in mechitzos