At this time every year, it becomes necessary to get outside and ride my mountain bike. On dirt.
In the month of April, the singletrack trail systems of our peninsula are invariably snow-covered. Even in years with less severe winters, they are too muddy to ride without damaging them. So to ride on dirt, at this time of year, means to ride on the dirt roads of this county.
Sunday afternoon I rode through Trap Rock Valley and through Laurium, with dirt under my tires on Angman Road, on Old Colony Road, and on Gas Plant Road. Water was rushing in the Traprock River, and in the ditches alongside Angman Road. Temperatures were somewhere in the mid-30s. The sky overhead was a somber gray.
My route started out in the valley, rolling past the fields still buried in deep snow. The sound of the gravel under my tires was satisfying, turning northward onto Old Colony Road. The ride up Old Colony is a long haul on a long grade, especially on my bike, which has no climbing gear and, in fact, has only one gear, period. Halfway up the hill, each downward pedal stroke could be appreciated as its own unique and isolated experience.
Old Colony Road is closed to automobile traffic during the spring thaw, so I was able to crank slowly up the middle of the road. A long, slow haul staring down at the road surface, with time and mind space to contemplate its composition.
The dirt roads of Houghton County are made up of a motley aggregate. One of its constituents is crushed mine rock, which the county road commission puts down to stabilize roads in the spring, according to Assistant Engineer John Cima. This is straight mine rock, screened down to one-inch size and smaller, with no clay added and with only incidental sand. This kind of rock was in evidence on Old Colony Road, Sunday, particularly at the edges.
When dirt roads are resurfaced, or new dirt roads built, the material used is a mixture known as 23A in the Michigan Department of Transportation's specification system. Sixty to 80 percent of the aggregate in 23A is three-eighths-inch diameter and smaller gravel; the remainder comprises gravel up to one inch in diameter and a proportion of clay.
County road commissions typically obtain their 23A aggregate from the gravel pit nearest the road they are working on. Thus, dirt roads tend to express something of their local geology. The gravel in Houghton County is glacial gravel, mostly black basalt and red sandstone. According to geologist Bill Rose, the glaciers also pulled down bits of granite and other types of rock from farther north, from Canada, but these rock types are few and far between in the gravel of the county. I did not identify any granite, for example, during my long climb up Old Colony Road.
Over time, gravel sinks into the sub-grade of the road, Cima said. This is particularly true in roads like Old Colony and Gas Plant, which traverse ground that is naturally high in clay content. Because of this, the road's underlying character is nearly always in evidence, no matter how much mine rock or 23A aggregate gets put down.
Red, local clay was the road surface's salient component on my ride down Gas Plant Road Sunday. There was no time to consider the composition of that road's surface as gravity pulled me toward the bottom of the valley at 35 miles per hour. But I brought home a generous sample of that good, red clay. It had rooster-tailed off my tires with an admixture of sand, coating my bike and myself. A souvenir of the (dirt) road.