Hard rock should erode more slowly than soft rock. Granite should resist weathering longer than sandstone. Mountains of resistant rock should stand taller and last longer than hills of weak rock. This is the intuitive model taught in introductory geology: rock strength controls erosion rate, and landscapes reflect the differential resistance of their substrate.
Measurements across diverse landscapes show that hard and soft rocks erode at similar rates when tectonic uplift is held constant. In actively uplifting mountain ranges, erosion rates measured by cosmogenic nuclides — beryllium-10 concentrations in river sediment, recording erosion rates averaged over thousands of years — converge regardless of rock type. Granite and shale, basalt and limestone, erode at roughly the same rate.
The mechanism is slope adjustment. Harder rock develops steeper slopes. Steeper slopes concentrate more gravitational energy per unit area, increasing the erosive power of water and debris. The landscape steepens until the erosion rate matches the uplift rate, regardless of rock strength. Softer rock achieves the same erosion rate on gentler slopes. The landscape adjusts its geometry until all rock types are eroding at the rate set by tectonics.
Rock strength controls landscape form, not erosion rate. Hard rock produces steep, rugged terrain. Soft rock produces gentle terrain. Both erode at the same rate. The difference is visible in topography — slopes, relief, roughness — but invisible in denudation rates.
The landscape is a machine that converts any rock strength into the same output by adjusting its own geometry. Hard rock doesn't resist erosion; it makes the erosion happen differently. The mountain isn't tall because its rock is strong. Its rock is steep because its rock is strong. The erosion rate is set from below, by tectonics. The surface shape adjusts to deliver it.