A BUMPY ROAD

Over the years at work I have been called to the counter to answer questions from Moorpark College students, who are enrolled in a geology class.  They want to know why the 118 Freeway west of Madera Road is "so bumby".  CalTrans fixes it, but the bumps return.  (You could write a script for a Hollywood thriller entitled "The return of the bumps!"  Well, maybe that wouldn't be such a good idea.)

The bumps are there because the freeway cuts cross the middle Sespe Formation, which is replete with what are referred to as "red" and "blue" beds.  You've no doubt noticed them.  Because of the strike and dip of the Sespe Formation (strike northeast-southwest and dip 30-40 degrees to the northwest), the beds cross under the freeway diagonally somewhat on end.  Unlike the predominately sandy strata of the Sespe, the red and blue beds are high in clay.  When they get wet, they expand - lifting whatever is on top, at least the first two or three feet.  So, up goes the concrete pavement on the road with each winter - cracking the concrete and providing a very interesting ride for vehicles on the road.  If you're traveling on the freeway, your right front wheel goes up first, followed shortly by your left front when wheel, then by your right rear wheel, and then by your left rear wheel.  Going down the other side of the bump, is the opposite of that same twisting effect.  If you watch the beds in the road cut, you will pass them, and then be twisted.  This is because of the northeast-southwest strike of the beds.  When the beds dry out to some degree, the road does flatten somewhat, but never goes back to where it was before the road was paved.

Every once in awhile CalTrans will grind down the concrete or apply some asphalt, only to have the worsened state return in one to three years, depending upon how much rain we get.  Had those road beds been provided with two more feet of road base when they were originally constructed, the problem might have been eliminated or greatly reduced.  That however would have required deviating from the standard construction plate - something that apparently was not considered at the time.

The Sespe Formation in this area is a non-marine deposit from a tropical forest environment of Eocene/Oligocene age - about 24 million years old and younger as you go west.  The red and blue beds are "overbed" strata formed when the river flooded adjacent river valley and deposited its clay load.  In the process, small animals and occasionally large mammals were drowned.  Mice, for example, were drowned in their burrows.  As a result, the Sespe Formation in this area has produced about half of the recorded vertebrate fossils known for the Oligocene period.

Mike Kuhn

6-28-98