From Granite Falls, the road is paved for the first 22 miles as it follows the South Fork of the Stillaguamish River to its headwaters at Barlow Pass, entering the Mt. Baker-Snoqualmie National Forest near Verlot. Numerous campgrounds and hiking trails offer diversions along the way, and access to three fabulous swaths of wild mountain country: Boulder River Wilderness , Henry M. Jackson Wilderness , and Glacier Peak Wilderness .
North of Barlow Pass, the road turns to gravel and becomes narrow and winding as it drops along the Sauk River. It remains gravel for 14 miles; the last seven miles to Darrington  are paved.
West of Darrington, the Mountain Loop Road passes through a wide valley bisected by the North Fork of the Stillaguamish River and filled with hay piles, beehives, horses, sheep, chickens, hogs, big red barns, cut-your-own Christmas tree farms, lumber mills, clear-cuts, and regenerating stands of trees. Openings provide glimpses of the snowy peaks that cap the Cascade Range.
The Darrington area is an anomaly in the Cascades. Whereas the Seattle  area receives 30–40 inches of rain per year, Darrington  gets an average of 80 inches, while Monte Cristo  gets over 140 inches, creating a dense rainforest much like those of the Olympics. The low elevation here means questionable snowfall, though cross-country skiing and snowmobiling are popular area activities.
It is imperative to carry adequate clothing while hiking in this part of the Cascades, even if you’re out on a short hike. The weather changes rapidly sometimes, and nearly every summer someone dies of hypothermia a short distance from his or her car.
The highway and logging roads are popular snowmobile routes in winter, but that portion of the road over 2,361-foot Barlow Pass is blocked by snow beyond Elliot Creek on the north side and Deer Creek on the south side from mid-November to mid-April.
Parts of today’s Mountain Loop Highway overlay trails that were used for centuries by the Native Americans who first inhabited these lands. The 1889 discovery of gold and silver in the Monte Cristo  area led to a mad rush of miners and others attempting to get rich quick. To transport the (assumed) mineral wealth, a railroad was constructed from Monte Cristo over Barlow Pass, down the canyon created by the South Fork of the Stillaguamish River, through the new town of Granite Falls , and on to Everett , where the ore would be refined.
Funded by an East Coast syndicate with financial backing from John D. Rockefeller, the railroad was poorly designed and subject to repeated flooding. These floods, combined with a financial depression in the 1890s, forced the mine owners to sell out to Rockefeller in 1899, and even he was compelled to give up when the ore petered out a few years later.
With cessation of mining, the Monte Cristo area began attracting tourists, and the trains turned to offering weekend excursions into the mountains. Business flourished in the 1920s, and hotels were added in Silverton and near Big Four Mountain to cater to the wealthy—the latter even featured a nine-hole golf course.
All this came to a screeching halt following the stock market crash of 1929. The railroad shut down in 1936, and the old railroad grade became an automobile road. Two years later, the CCC began construction of a narrow mountain route north from Barlow Pass to Darrington , completing the final link in today’s Mountain Loop Highway.