The Stair-Step Hills

The hills along this stretch of US 281 all have the same profile. A flat bench, a short steep riser, another bench, and every hill for miles repeating the same steps at the same heights. That is the Glen Rose Formation, a shallow-sea deposit about 110 million years old, laid down as alternating layers of hard limestone and soft marl. The marl erodes and the limestone does not, so what is left on every slope is a staircase. Because the layers lie almost flat and run continuously, a single step can be traced across the landscape for miles without changing elevation. Brushy Top, the 1,631-foot hill immediately west of the road here, is a good place to count them. Individual benches have been traced across this country for many miles without any apparent change in expression. The same layering explains the water. Rain soaks into a porous limestone bench and travels sideways until it meets an impermeable marl bed, then emerges as a seep or a small spring along the hillside. That is why Hill Country springs so often line up at one elevation right across a valley, and why a ranch can have reliable water on one bench and none fifty feet lower. This high ground is also a divide. Blanco County drains two ways. The northern two thirds runs to the Colorado by way of Miller Creek, Cypress Creek and the Pedernales, and the southern third runs to the Guadalupe by way of the Blanco and the Little Blanco. Driving south through these hills you cross from one river basin into the other.