Numerous old river valleys and furrows entrenched in.
The Columbia river has entrenched itself in a canyon-like valley around the northern and Western side of the lava plains; Snake river has cut a deeper canyon farther south-east where the plains are higher and has disclosed the many lava sheets which build up the plains, occasionally revealing a buried mountain in which the superposed river has cut an even narrower canyon.
But at length the frontier was passed, and Soult forced back into his entrenched camp at Bayonne.
As the French columns coming up from the south and west gradually surrounded him, he drew in his troops under shelter of the fortress and its improvised entrenched camp, and on the 15th he found himself completely surrounded.
Murat attacked the Russians, who had halted in their entrenched position, on the 11th and drove in their outposts, but did not discover the entrenchments.