Pl. Tim. 25.d.6:
ἥ τε Ἀτλαντὶς νῆσος ὡσαύτως κατὰ τῆς θαλάττης δῦσα ἠφανίσθη˙ διὸ καὶ νῦν ἄπορον καὶ ἀδιερεύνητον γέγονεν τοὐκεῖ πέλαγος, πηλοὺ κάρτα βραχέος ἐμποδὼν ὄντος, ὅν ἡ νῆσος ἱζομένη παρέσχετο.
What does the phrase “πηλοὺ κάρτα βραχέος” mean exactly?
Well. That is a bit odd. So I had to go digging, and as often, there’s an old schoolboy commentary with the Borg (google books). From R.D. Archer-Hind’s edition —
πηλοῦ κάρτα βραχέος > I believe this reading to be perfectly correct, although I am unable to produce an exact parallel. βραχέα was the regular word for shoals: cf. Herodotus II 102 θάλασσαν οὐκέτι πλωτὴν ὑπὸ βραχέων : also IV 179, and Plutarch > de genio Socratis > § 22 ἀραιὰ τενάγη καὶ βραχέα. The peculiarity in our passage is of course that βραχέος is an adjective agreeing with πηλοῦ. But though this use does not seem to occur elsewhere, I see no conclusive reason for rejecting it here; and certainly no tolerable substitute has been offered for it. […] Accordingly I retain πηλοῦ κάρτα βραχέος in the sense of `very shoaly mud’.
hi, in addition to the herodotus refs above, herodotus uses the exact words κάρτα βραχέος when describing a small space at the top of chests filled with stones (and gold is to be laid in this small space on top of the stones), see 3.123:
http://www.perseus.tufts.edu/hopper/text?doc=Hdt.+3.123&fromdoc=Perseus%3Atext%3A1999.01.0125
it looks like plato is referring to the depth of the πηλός in the same way.
cheers, chad 
Is τοὐκεῖ crasis for τὸ ἐκεῖ which functions as the subject of the verb - “what is there has become an impassable and uninvestigated flooded plain”?
If so, does the following clause function as absolute genitive?
Thanks,
Nate.
Yes.
which functions as the subject of the verb
Not quite. πέλαγος εος τό (open) sea. So, ἐκεῖ is attributive, “the sea there, the sea at that place,” κτλ.
If so, does the following clause function as absolute genitive?
Yep.
My English is not “refreshed” for some time… Is it “shoal”= shallow, an adjective? I understand "πηλὸς κάρτα βραχὺς "like “reef”, very shallow sea-bottom…
shoal can be an adjective as well as a noun (even a verb but it isn’t a very common verb by any means.)
thank you all for your replies!