floodplain are relatively young in geological terms, and owe their existence to tectonic forces along the San Andreas Fault (the boundary between the North American Plate and Pacific Plate) and its subsidiary fault and fracture zones. The San Gabriel Mountains are a fault block mountain range, essentially a massive chunk of bedrock dislocated from the North American Plate and lifted up by movement along the San Andreas. The rock is mostly of Mesozoic origin (65–245 million years old) but the deepest layers are up to 4 billion years old. However, the uplift of the present mountain range did not start until about 6 million years ago. The mountains are still rising as much as per year due to tectonic action along the San Andreas Fault. The Puente and Montebello hills are even younger, no more than 1.8 million years old. As the hills formed the San Gabriel River maintained its original course, cutting the water gap of the Whittier Narrows. Composed of ancient, highly fractured and unstable crystalline rock, the San Gabriel mountains are subject to tremendous amounts of erosion. Rapid erosion caused by heavy winter storms has created the dramatic canyons of the San Gabriel River. In the headwaters, streams often follow fault traces; the West Fork and part of the East Fork run along the San Gabriel Canyon Fault, which extends in a nearly straight line from east to west across the center of the San Gabriel Mountains. In the winter, the mountain regions are prone to landslides and destructive debris flows which has required the construction of many debris basins to protect foothill communities such as Glendora and Monrovia, but these works have not always been effective during the biggest storms. During floods the river transports large volumes of sediment from the mountains into the San Gabriel Valley, ranging from fine sands, gravels, clays and silt to car-sized boulders. Starting in the Pliocene, about 5 million years ago, the Los Angeles Basin experienced considerable tectonic subsidence; at the same time the San Gabriel River was depositing a huge alluvial fan, essentially an inland delta, radiating from the mouth of San Gabriel Canyon. This has combined with smaller alluvial fans from other drainages along the front range of the San Gabriels to form the flat valley floor. In the San Gabriel Valley, riverine alluvium deposits can be up to deep. On the coastal plain, San Gabriel River sediments