Milton 12

Rosenwald L of C
Milton Plate 45

Milton is holding up Urizen; they have crossed the Arnon:

From the Blake Dictionary we get an idea of Arnon in the Bible.

From Bible in Milton
Milton, Plate 19 [21], (E 112)
"Urizen emerged from his Rocky Form & from his Snows,

And he also darkend his brows: freezing dark rocks between
The footsteps. and infixing deep the feet in marble beds:
That Milton labourd with his journey, & his feet bled sore
Upon the clay now chang'd to marble; also Urizen rose,
And met him on the shores of Arnon; & by the streams of the brooks

Silent they met, and silent strove among the streams, of Arnon
Even to Mahanaim , when with cold hand Urizen stoop'd down
And took up water from the river Jordan: pouring on
To Miltons brain the icy fluid from his broad cold palm.
But Milton took of the red clay of Succoth , moulding it with care
Between his palms: and filling up the furrows of many years
Beginning at the feet of Urizen, and on the bones
creating new flesh on the Demon cold, and building him,
As with new clay a Human form in the Valley of Beth Peor."

At other places in 'Blake' we learn that Los  got the better of Urizen and learned to love his greatest enemy.

Arnon:

Arnon means "rushing torrent". The Arnon was a river rising in the mountains of Gilead, E of the Jordan, and reaching the Dead Sea through a stony and precipitous chasm of red and yellow sandstone. The name is also applied to the valley, or valleys, (Num 21:13,26; Deut 3:8; Josh 12:1; Judg 11:22; Isa 16:2; Jer 48:20).