The mapped area lies along the Motagua Fault Zone, a long linear feature associated with the Bartlett Trough in the Caribbean. Block faulted east-west ridges and valleys form the topography of the region. The Sierra de Las Minas, a complex of metamorphic rock, lie north of El Progreso. At the foot of these hills, the fault zone follows a serpentine horst which underlies the Motagua Valley. A long fault has divided the horst into northern and southern blocks. The southern block has been repeatedly depressed in the past, forming a trough in which two successive clastic units have accumulated. The older of these, the Subinal Formation, was previously defined by Hirschmann (1963), but is here further described, discussed, and dated. The younger deposit, the Guastatoya Series, is defined herein for the first time. The southern portion of the horst is itself divided by faulting into a northern ridge of Subinal material and a southern valley occupied by the Rio Guastatoya and the clastics of the Guastatoya Series. South of the valley is Cerro Piedra de Cal, a ridge consiting primarily of limestone, which is bordered on the south by a second metamorphic complex. Several volcanic and recent alluvial and landslide deposits cover portions of the older units in the valleys and on low hills in the area. The fault zone contains numerous large and small faults and associated jointing with various orientations. It has been the site of extreme disturbance with local volcanism in the past, and some faults are still active. Most of the observed recent movement in the valley has been vertical, but an attempt was made to determine if there had been strike-slip motion there in the past. Nothing was found to confirm or deny the existence of such motion on a large or small scale when the fault was younger.