All I remember about this book is that a group of kids (who were maybe related. There were two brothers?) ended up in this absolutely empty valley - there was a plane involved. And they explore this valley - find a cave to live in. And at some point they solve a mystery - a chicken is involved - and they find gold. And then get rescued. This was is the Hardy Boys/Nancy Drew genre. All I remember about the cover is that it had a numeral of sorts on it. Also, in another book in the same series one of the brothers finds a rare bird - a great auk or something.