Landslide on the eastern side of Kulon Progo Mountains was triggered by claystone from Nanggulan Formation, which contains montmorillonite and saponite. This clay is expansive and can be used as a sliding plane. The pattern of claystone subsurface is assumed to match the surface. Purpose of the research was to figure out the depth and pattern of claystone. Research method used was Electrical Resistivity Tomography (ERT) in a Dipole-Dipole configuration. ERT measurements were conducted on two lines, each 300 meters long, with an electrode spacing (a) of 10 meters and layers (n) of 7. Data processing was performed using Res2Dinv. ERT measurements revealed evidence of ground disturbance (bumpy roads, subsidence, and shifting); the mudstone is dark, slippery, impermeable, and contains volcanic material. Mudstone has a resistivity of <30 ohm.m, sandstone ranges from 31 to 190 ohm.m, and igneous rock >190 ohm.m. Line 2 is made entirely of mudstone, whereas line 1 is made up of claystone, sandstone, and breccia, or igneous rock boulders. The claystone, acting as a sliding plane, generated the landslide, which continued westward and reached depths of up to 25 meters.
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