Spectacular cliffs are an important part of the scenery at Torrey Pines State Natural Reserve. The land must descend somehow from the 100 yard altitude at the lodge to sea level at the beach. The view of the Reserve from Del Mar shows a gradual steepening of the slope to the beach to about 45 degrees halfway down. Then the land plunges vertically to the sand. What is needed to produce an almost vertical cliff rather than just a steep slope of 45 to 60 degrees?
The first requirement is for layers of rock strong enough and thick enough to stand up in a cliff. If we dig a trench in weak soil, the walls will collapse when it is a few feet deep. Even in strong soil, OSHA requires shoring of trench walls to prevent collapse on workmen. The Delmar Formation, Torrey Sandstone, and Lindavista Formation are all strong enough but the Lindavista is not thick enough to make much of a cliff. The more recent Bay Point Formation is not strong enough to form a very high cliff so it erodes into our ‘badlands” topography with very steep slopes but no cliffs higher than 20 or 30 feet.
The second requirement is for a mechanism to cut the cliff. The mechanism was man’s machinery in our road cuts by the Fleming trail and on North Torrey Pines road. In several of our canyons, streams cut the rock, most notably for the cliff in the Torrey Sandstone opposite the Rim trail. Our biggest and best cliff is along the beach where the waves cut out the bottom of the cliff. The cutting process occurs mostly at high tide during winter storms. The hard cobbles on the beach are thrown against the cliff by the million. The cliff top falls when the undercut is too great. If you look up from a recent rock fall, you will usually see an arch in the cliff face. The arch shape is self-supporting even if the rock above it is cracked.
The third requirement for a cliff to exist is that debris must be prevented from accumulating enough to bury it. Debris comes from rock falls from the cliffs and from soils washed down from the slopes above the cliffs. A hard, nearly level cap rock at the top of a cliff will reduce the wash debris and protect the softer rock below. The Lindavista Formation is the cap rock for our road cuts. The old beach cliffs at various elevations in the Reserve have been buried so well that they are hard to see at all. In our road cuts, debris removal is by trucks. In the canyons, streams remove debris during floods. On the beach, debris is removed by storm waves during high tides.Cliff erosion is also caused by water from rain or such human uses as lawn watering draining into cracks behind the cliff. Friction along the cracks is reduced and landslides result. When you see water running out of cracks in a cliff, stay back.
Next topic: Erosion