The Big Hole was rightly named by settlers, as it is the highest and widest of the broad mountain valleys of western Montana. The valley separates the Pioneer Mountains along its eastern margin from the southern Bitterroot Range on the west. From the top of the howitzer trail in the park, visitors are able to see the broad expanse of the valley and the surrounding mountain ranges.
Like the Bitterroot Valley to its northwest, geologists explain that the Big Hole is probably the gap opened behind a big block of the Earth’s crust (the Earth’s outer skin composed of granite and similar rocks in the continents, or basalt in the oceans) that detached from the top of the Idaho Batholith (a large mass of intrusive granite or similar rock that covers an area greater than about 40 square miles in northern Idaho and northwestern Montana) and moved east about 70 million years ago. The Pioneer Range along the east side of the Big Hole is that block.