Hysterical marker

I suppose a reader can figure it out, but why didn’t someone proofread the first part of this “Official Scenic Hisoric Marker” before it was installed?

Valle Grande Marker

About one million years ago, the magnificent valley
before you was formed by collapse, after a series of
tremendous volcanic eruptions ejected a Volume of
material more than 500 times greater than the May
1980 eruptions of Mt. St Helens, This event climaxed
more than 13 million years of volcanism in the Jemez
Mountains. Minor volumes of magma, leaking to the
surface as recently as 50,000 years ago, formed the
dome-like hills between  you and the skyline to the
north, which is the opposite wall of the enourmous
Valles Caldera. The heat from young volcanism
makes this area attractive for geothermal energy.

Even if the comma after “Helens” is translated as a period, that’s a 40-word sentence on a sign. I can’t even count all the dependent clauses.